I'd say you're cherry picking the exact examples where a radically left position ultimately became mainstream. That strongly implies that radical left positions are correct, fruitful and destined to become the norm if only given enough time.

In reality it's much more of a mixed bag. Some ideas are impractical, downright destructive or achieve the exact opposite.

Unappealing unhinged ideas that alienate large groups and act as the perfect recruitment tool for the right wing? Absolutely the fault of leftists.

But agree to disagree, this isn't the place.