I'm curious whether they are an overwhelming majority here, or just the largest plurality out of several, and not actually a representative of most posters.
Don't get me wrong, Marx definitely made some major achievements, got a ton of stuff 100% spot on, quite impressively that still measures up after 150+ years. At the same time, I think a lot has been discovered and researched in that period of time that makes me doubt some significantly foundational aspects of it.
Part of what might make this difficult is pinning down exactly what it means to be Marxist, esp so since most are brought up being taught complete nonsense about it. I'd probably boil it down to "The Materialist Dialectical view of History as being driven by the conflict between social classes (Ruling Class vs Working Class)". If you think I'm way off base here, feel free to downvote away and/or bully, shame, mock and/or troll me, but also please do so while teaching me a better/more accurate definition.
And I also really want to stress this isn't disparaging Marx, I just don't think he had the right tools available in his time to come up with what I'd see as a more valid foundation. Given another 100 or so years, an the advent and maturity of things like Systems Theory, Chaos Theory, Information Science, Quantum Physics, Sociology (which Marx could easily be considered one of the founders of) I could see his output being much more agreeable with me.
And of course, the almost dogmatic devotion later thinkers would have defending its scientificity (is that a word?) that practically bordered on fanaticism doesn't do any favors, but I try hard not to let what later people would do to his ideas affect my view of them.
Have a look at Okishio's theorem for an example of a twentieth century Marxist using modern mathematics to have a fresh look at some of Marx' ideas. Marxism isn't as dogmatic as you seem to think, how else could the huge differences between say Trotskyists and Stalinists exist; and certainly the dialectical view of history (wasn't that mostly Engels' work?) is something you won't see defended by many modern communists. People like Antonio Negri or Alain Badiou stray very far from orthodoxy, while still calling themselves communists.
Counterthesis: okishio’s theorem is dumb
Why would this be the case? It seems to me that at best it provides an exception, but not even really that because it's a "tendential" fall in the rate of profit and the capacity to innovate is not limitless.