What exactly changed so much since Marx to render him irrelevant?
You keep seeing "it's been 180 years!" and? A lot of theories being employed after millennias. Just throwing a number out isn't an argument, it's hand waving, "look how big this number is" trying to win over not with reason, but with vibes.
One of the posts being up "crisis of Marxism" which was coined in 1890s to argue the same thing: "omg you're still on it? It's actually gone, it's uncool, it's irreverent" 20 years later you have entire wave of revolutions throughout Europe.
"Workers united by race and nationality, not by class" yeah after bourgeois government created concentration camps for socialists, communists and trade unionists. Or were they empty? If class consciousness wasn't growing there would be no one to imprison, no efforts from capitalist class to divert popular ideology away from proletarian internationalism.
"Why Russia? It was not developed!" Nor did it have socialist transformation. Revolution in Russia didn't achieve much more than 1848 French revolution, it just so happened to occur while communist ideas were popular. It was majority peasant population overthrowing monarchy. Sure, it had radical elements early on, so did French one and in both cases they were swept away by following reaction.
Bolsheviks never saw Russia as socialist, it was materially ready to ditch feudal monarchy with ideological communist aspirations of countries that already have. It was possible with the aid of the developed countries, but once revolution in Germany died, so did Russian near communist future.