Accepted by economists? I'm no expert on economics or economists' opinions but I'll say this, Marx predicted ever falling wages for all except an ever smaller number of capitalists. He was blatantly wrong about that as was pointed out as early as the late nineteenth century by Bernstein. Standards of living for average people may have stagnated in many advanced Western economies over recent years but they are still far, far, higher than they were in Marx's time.  The only part of Marx's take on capitalism that I've seen praised in recent years is his highly impressionistic description of globalization in the Manifesto - and that's not really a 'critique', if anything the opposite.  Like the good child of the Enlightenment that he was Marx believed in Progress, he saw capitalism as the chief motor of Progress.  It's just that his deterministic version of historical teleology argued that capitalism would inevitably collapse and turn into socialism.

I also don't understand what you  mean by capitalism never existing. If by that you mean that the abstract model has never existed - sure. But no model ever does, they are just heuristic tools. In the messy reality of this world we have had various forms of capitalism which have functioned in different ways.