If I got the grand theme of "neo-realism" right (I and I might have not), every somewhat big country has just one primary concern: that some other country is going to become the hegemon. (The position statement appears to have change slightly, but inconsequentially over 20 years; in 2001 Germany was described as a possible hegemon, at least in Europe, while in 2018, it was now being ruled out, and so was Russia. Whereas China is described as the perennial candidate for supplanting the US as the world hegemon.)
In the latter talk, at one point Mearsheimer says that Russia too is or should be worried about China becoming a hegemon. But what I find interestingly absent (or at least not easy to find) in Mearsheimer's writings and speeches is any argument why the whole world is not uniting against the United States... which after all he admits is the present hegemon. He does mention that Iran is allying with China against the US. But, according to neo-realism, what is preventing a world-wide alliance against the United States hegemony? (Ideological disagreements would be an obvious answer, but ideology never seems to be mentioned by "neo-realists" as a force shaping alliances or competition, unless I've missed those lectures/writings.)