I don't think it would be fair to call them the "pinnacle" (there were many things they could not do or know then that we can do or know now), but it is the case that the Cold War period is considered something of a "Golden Age" of physics in the United States in particular. Between 1945 and the early 1970s there was practically an unlimited spigot of government money, and an unsaturated need for "scientific manpower," with this new influx of money and talent a huge number of remarkable feats were accomplished, the likes of which that have been hard to replicate since then. Daniel Kevles profiles this period very well in his book The Physicists, and talks about the conditions that led to it. Largely it involved an influx of refugees from war-torn Europe, coupled with new US government commitments to dominance in science, plus a newfound appreciation of the role that even basic science could play in wartime success. This started in the WWII period but really hit its peak after Sputnik (1957).

From the 1970s onward, that growth seems to have dampened. Part of this was that the funding situation changed: the The Mansfield Amendment of 1969 put heavy restrictions on how the Department of Defense could fund science, essentially eliminating them as a funder of basic research. Deep cuts in defense funding further reduced the poor, as did economic strife generally. The market for PhD physicists crashed, and everything got a lot tighter. And it turned out that there was a saturation point for this kind of talent and work. David Kaiser's article, "Cold War requisitions, scientific manpower, and the production of American physicists after World War II", goes over this in a lot of detail.

By the 1990s, US federal funding on science megaprojects was considerably reduced, so that now the biggest physics megaprojects are transnational ones. It may also be the case that the "low-hanging fruit" of physics was picked in that earlier period — leaving the "big new ideas" into more nebulous, hard-to-test or untestable realms. As a result it seems to require a lot more work to get any kind if significant "progress" than it did during that "Golden Age," separate from the fact that the funding situation has shifted dramatically. My colleague John Horgan's The End of Science is an interesting, if controversial, look at the question of whether we've run out of "revolutions." Whatever one thinks on that, it does seem like the rate of progress has diminished, whether that is because it would inherently do that after awhile (as some commentators on science argued it would even during the "Golden Age") or whether it is because of changed conditions (which is sort of how I lean — what are the odds that we figured everything easy out by the 1990s, right when the funding context dramatically changed?).

A retired physicist colleague of mine, who got his BS from MIT in 1957 and his PhD from Columbia in early 1960s, ruefully observed not long ago that in his day, the smartest kids all wanted to be physicists; in the 1990s, they wanted to be biologists; today, they all go into quantitative finance. This is certainly the case at the STEM school in which we teach — high student debt coupled with economic uncertainty equals many good students pursuing the option that they think will lead to maximum economic security. That the context of science matters is a truism for historians of science; it's not implausible that we've made a world that has slower work in these areas.