Standard historian opening: nothing in history is inevitable. Things only look inevitable with the benefit of hindsight. Anyhoo! My answer below comes from my knowledge as Cold War historian, not a Soviet specialist. I hope that it goes some way towards answering your question. Regarding your query about George Kennan, I'm afraid you'll need to be more specific about which article you're referring to! He was a prolific writer and thinker, and produced a vast body of work.

The Cold War ended because of a) a group of reformers in the USSR; b) changes to Soviet policy (at home and abroad); c) and the efforts of millions of people in the Soviet Union and Eastern Europe. The Soviets finally abandoned the war in Afghanistan, released their grip on Eastern European 'satellite states', and allowed the independence of the Soviet republics. Senior Soviet officials (especially Mikhail Gorbachev) also made some pretty remarkable (in the context of what had gone before) concessions in terms of their attempts to halt (and potentially reverse) the nuclear arms race. The Cold War ended when it did and how it did because of series of events that brought into power a Soviet leader with a radically different mind-set, allied to the pre-existing demands for change that had sweeping through Eastern Europe for quite some time (indeed, it's possible to argue that these demands had always been there, but by the 1980s they were being voiced more fully and by a greater proportion of the population). The combination of reform from the top and 'revolution from below' (although to what extent events in Eastern Europe were 'revolutions from below' is something that needs to be analysed for each individual case, e.g.: the manifest differences between what happens in Poland and East Germany) were what brought the Cold War to a (largely) peaceful conclusion.

As détente staggered towards collapse in the mid to late 1970s, so too did the Soviet dream. Although the USSR still remained a superpower in terms of its nuclear arsenal and gigantic conventional forces, it lagged behind the USA in pretty much every other measure of military power. The USSR was also being seriously hampered by a number of internal factors:

1. It was led by a group of ageing, sick old men, wedded to increasingly outdated Marxist-Leninist dogma. Leonid Brezhnev died in November 1982, to be succeed by former KGB chief Iiuri Andropov who died in February 1984, who was then succeeded by Konstantin Chernenko who died in March 1985. At the top, there was no innovation or desire for any kind of change to the increasingly untenable and unstable status quo.
2. The Soviet economy was in a truly terrible state, as the military-industrial defence complex absorbed vast amount of capital and resources. Consumer goods were sparse, queues for food common, and it had become clear to millions of Soviet citizens that Moscow’s tales of how things were much worse in the west were nothing more than obfuscation at best, outright lies at worst.
3. Soviet satellite states in Eastern Europe were becoming increasingly restive, with demands for greater political, social, and cultural freedoms. And even further afield, Afghanistan was a running sore, consuming personnel and resources in the seemingly unwinnable war against the mujahedeen.

In the mid-1980s, some truly dramatic changes took place. The most important in terms of national leadership was the ascent of Mikhail Gorbachev to the top position in the Soviet Union. This was coupled to US president Ronald Reagan’s increasingly conciliatory language and actions, a radical departure from the confrontational tone of his first term in office.

It became clear to the Soviet politburo that they could not continue with the merry-go-round of leadership, with ill old men continually reaching the top and then dying in short order. On March 11, 1985, one day after Konstantin Chernenko's death, Gorbachev became general secretary of the Communist Party of the Soviet Union (CPSU) and leader of the USSR. Prior to to coming to power, Gorbachev had been identified by Western leaders – particularly the UK's Margaret Thatcher – as a high Soviet official that they could work with. Setting aside dogma, Gorbachev was willing to debate and discuss in good faith.

Importantly, Gorbachev came to power as a reformer, not a revolutionary. His reformist policies can be summed up in two words. Glasnost was his policy of openness, increased transparency with Soviet government, and a willingness to tolerate dissent and discussion. Perestroika was reform of the USSR’s politics and economy, but within a socialist system. This last bit is crucial, as it was not Gorbachev’s intention to turn the USSR into a capitalist state or to see it cease to exist. Most importantly of all, Gorbachev was keen to engage in constructive talks with the United States, and he found a willing partner in Reagan. Gorbachev offered concessions that were even more radical than outside observers could have predicted.

Moreover, his implicit and then explicit renunciation of the Brezhnev Doctrine (the Soviet right to militarily intervene if communist rule in satellite states was threatened), his willingness to allow domestic debate about reform, and his openness to new ways of thinking sent a powerful message that this was not the same old same old Soviet Union. Indeed, his inaction when it came to the changes that swept Eastern Europe in the late 1980s (end of communist rule in Poland, for example) said more than any amount of explicit statements and speeches.

What happened from the late 1989 fall of the Berlin Wall onwards was a series of events that cast a long shadow into the twenty-first century. Reagan’s successor George H W Bush agreed with his West German counterpart Helmut Kohl that a speedy push for German reunification was a wise move. Things moved with astonishing rapidity, with monetary, political, and military reunification being agreed in short order. With the 1990 reunification, the whole of Germany could finally be made part of the European project. With the Maastricht Treaty's conclusion in 1992, the European Community metamorphosed into the European Union, with a reunified Germany as a key component.

Perhaps even more dramatic than Germany's reunification was the USSR’s collapse. Gorbachev’s willingness to let the states of eastern Europe go their own way predictably (with a degree of hindsight!) led to the USSR's constituent republics demanding their right to self-determination. Weakened by economic decline and ever deeper political divisions at both the centre and the periphery of Soviet power, high officials in the Kremlin could do little to avoid this. What was remarkable about the USSR’s collapse is its speed and its (mostly) peaceful nature. First to go were the Baltic states of Latvia, Lithuania, and Estonia. After a futile attempt to keep them on board, the Baltic states went their own way and achieved independence.

This led to significant domestic criticism of Gorbachev from harder-line communists who blamed him for ‘losing’ Eastern Europe and giving away what remained of Soviet power. After a failed hard-line coup in August 1991, Gorbachev just couldn't salvage the reformed Soviet regime or get any kind of agreement for a looser confederation. The Russian leader Boris Yeltsin and his allies – who had been instrumental in defeating the August coup – won out, and in November the CPSU was banned and in December, the USSR was superseded by the short-lived Commonwealth of Independent States (CIS). The nations that had made up the Soviet Union now went merrily on their own ways, to varying degrees. The Cold War was over, as one side simply ceased to exist.

In the end, what role did the high level leaderships in the USA and USSR play in bringing about the Cold War’s end? US presidents – Reagan and Bush – played an important, but crucially secondary role. Reagan became more conciliatory in his second term, but Gorbachev revolutionised the USSR’s foreign policy. George Bush supported the Soviet leader, but his generally cautious approach towards events in central and eastern Europe was overshadowed by Gorbachev’s manifest boldness. The changes to Soviet policy were of a greater magnitude – and were far more painful for the USSR as a whole – than were any changes to US policy. The Cold War ended when it did because of series of events that brought into power and Soviet leader with a radically different mind-set. Once in power, Gorbachev drew upon ideas that were perhaps not necessarily that radical in a broader, global sense but were fundamentally radical within the framework of what had gone before in the Soviet Union. These ideas permitted the inauguration of conceptual and systemic change on the domestic and international scenes. Coupled to the vocal desire for changes in Poland, East Germany, Hungary, Romania, and elsewhere, the Cold War’s end fundamentally came from the east, not from the west.

Hope this answers your question.