After reading a few biographies of Stalin and watching the very excellent BBC documentary "Stalin: Inside the Terror", I have been thinking a great deal about the unique nature of his rule. The constant terror he employed both on Russian society at large and the Communist elite, never allowed for a permanent ruling class or institutions to develop. Almost all of his close associates became victims of political purges and the people surrounding him at any given time were always part of a transient group, in constant fear for their lives. The old Bolsheviks were almost all executed by the end of the 30's, including men like Nikolai Bukharin who helped him to power. Much of Stalin's own extended family was killed off or imprisoned including the relatives of both of his wives. Even the leaders of his secret police and the Gulag system became victims of the purges.There was was little of the nepotism and vertical patronage that characterizes most authoritarian regimes, even Hitler was loyal to his cronies and those who had been with him since the early days of Nazism. He also allied himself with industrialists and the old Prussian elites in the Wermarcht, giving both groups many tangible benefits. In contrast there never seemed to be a solid base for Stalin’s power, instead there was just constant paranoid terror and upheaval. Stalin himself was the only constant and became more than any other dictator the absolute and unchallenged embodiment of the state. How did Stalin maintain such absolute control over the USSR when even total loyalty to him was never rewarded in the long term?
I would suggest that the degree of terror/destruction might be slightly overstated in your narrative. For example the Old Bolsheviks were pretty well wiped out by the purges etc. but consider that many of Stalin's own allies stuck with him for decades, like Molotov and Kaganovich. Figures like Bukharin, Zinoviev etc. were essentially independent figures, who were only temporary allies with Stalin by contrast. Basically what I'm saying is some of Stalin's guys did get purged, but not all of them and keep in mind that the turn over wasn't immediate.
You might also consider that there were mechanisms of a sort in place for replacing purged leaders, even if they were not entirely effective. So consider the case of Yagoda, Yezhov and Beria. Yagoda was bumped off, but not before Yezhov was established as a successor (whether by his own efforts, a set up or both is hard to say), and similarly Beria was firmly established before Yezhov was eased out then purged from the NKVD. There were even signs that Beria was in trouble in the post WWII era with the emergence of counter-figures like Abakumov in the security apparatus. Even outside of the NKVD you can see similar patterns in other areas, Khrushchev's rise for example was very much the result of his getting jobs 'vacated' by people who got arrested (or moved on to bigger and better things for that matter) and in the lead up to these promotions he was given opportunities to prove himself. These transitional mechanisms were not perfect, some of the replacements were just not up to the job. Alec Nove argued that the short falls of the 2nd Five Year Plan lay with the loss of experienced managers, engineers etc. from the Great Purge. Similarly there is also an argument for the Red Army's failures in 1940-1941 being a result of a the upper levels of commander being cleared out (though the rapid expansion of the Red Army was also a contributing factor).
Cronyism etc. was certainly still present, but it was a question of scale. Local leaders certainly did look after their own and had their own patrons in the 'centre'. Stalin's big boozy dinners for his inner circle is also of course a classic symbol (symptom?) of cronyism. But compared to the later more entrenched networks apparent in the Brezhnev era it was relatively minor - arguably one of the impacts of the purges and resulting sense of fear was bosses tended to keep that sort of thing on a more 'reasonable' level.
Source-wise you might get a better feel for the changes in environment and leadership style from a biography of Stalin, such as Robert Service's or Simon Montefiore. I think it was Service who portrayed Stalin in the post-WWII era as being a bit less likely to use terror/purges, and more hands off. In a way, late Stalin was a prelude to the Brezhnev era. You might also like to take a look at J. Arch Getty's Road to Terror as an alternative take on the purges, looking at internal party dynamics and the internal logic of the purges. It also highlights the degree to which local bosses were running (or trying to run) their own show. Alec Nove's argument for the failures of the 2nd Five Year Plan can be found in his Economic History of the Soviet Union 1917-1991.
You should also look at Sheila Fitzpatrick's classic, The Russian Revolution. One of the main arguments not already mentioned is about how many people owed their positions to Stalin because they rose up quickly as replacements of purge victims.
And remember the very real fear of infiltration/invasion by either fascist or capitalist powers, and fear of internal "enemies" of socialism. Even as people disappeared all around them many continued to believe that most of them must have been guilty of something, because it was perfectly plausible at the time that the state was indeed being constantly besieged by "enemies."