If you said socialism you might have a good point. Marxism is a theory for one variant of socialism, but it's not the only one.
Conceivably, all Marx's predictions could be proven untrue and rejected, but there would still be some kind of socialism or some sort of anti-capitalist impulse.
It reminds me of the routine about Halloween costumes on Community:
Troy: I'm a dracula.
Abed: You mean a vampire?
Troy: I don't need to know which dracula I am to be a dracula.
Marxists are socialists, but I don't think all socialists are necessarily Marxists.
OK, point taken, but it is essentially the same system that led to 100 million innocent civilians in the 20th century being put to death by one way or another.
The ends justify the means. The good of the majority outweigh the rights of the few...or the one.
I am corrected. But my point stands. And I think it is historically indisputable.