The "end of history" bit is Hegel, really. I could see Marx being considered Orthodox in some History and English departments, but his viewpoint is definitely a minority among Economists, and society is not broadly structured in a way that reflects his thinking. The social nets we see today come nowhere near a from-each/to-each narrative. The workers do not own the means of production. Wealth is more concentrated than ever.

e.g. the most prominent US democratic candidates openly support ideas that are more marxist than anything else. They represent a lot of people, it's not some small heresy

That party is currently in the minority, but more importantly their policies do not reflect Marxism. Their last Presidential nominee and last President both supported capitalism. You can point to some figures that pay lip-service to socialism, like Sanders and Ocasio-Cortez, but even then the details of their policies are a mixed-market capitalism with a bigger socialist band-aide than we currently have. Marx was suggesting that people would identify as a groupman and act as if everybody mutually possessed everything. Obama (who was widely accused of being a communist) forced people to purchase healthcare from private corporations or face a government-imposed fine. These things are wildly different. While I wouldn't argue that the Soviet Union was ever really embodying Marxism either, the idea was certainly orthodox. If the Democrats widely identified as Marxists despite not actually implementing Marxist policies, I'd give you that Marxism was orthodox in their party (and simply call them hyprocrits in addition). This is not the case. They both widely denounce Marxism and don't implement Marxist policies. We can't call every welfare program Marxism unless we're willing to retroactively label Emperor Trajan a Marxist; Marxism is more specific than welfare.