Far too many people have actually studied Marx in academic situations for Marxist economics to ever become mainstream at the very least. The simple fact that the Labor Theory of Value is fundamentally incorrect would be the major problem. We aren't adopting Marxist economics like we won't be going back to a Lamarkian perspective on evolution, because we know it is wrong.
Marxist sociology is what I hope becomes more common.
Edit: Marx (and similar theorists before him) do a better job describing Capitalism compared to those whose paychecks depend on them not understanding / distorting it.