how long did it take Europe to shed absolute monarchism? Centuries

Yes, but history progresses at accelerating rates as societies develop. Humanity spent tens of thousands of years being hunters-gatherers, the neolithic and agricultural revolution brought forth much faster changes, then feudalism, and ultimately capitalism and the industrial revolution.

The USSR was a nice idea, but it took less than half a century for it to succumb to bureaucratic corruption

I'm not sure what you're talking about, care to explain?

Arguably, it was even deeper autocratic corruption than in Western democracies.

Only arguably so if you go against material evidence, in my opinion. Universal free healthcare and education to the highest level, guaranteed right to housing, abolishment of unemployment, high level of worker rights, respect and promotion of the local cultures and languages to an extent unparalleled until that point in history, women's rights (more Soviet female engineers by the 1960s than in the rest of the world combined and highest rates of female representation in organs of power), extremely high rates of unionization, quality of public transit and urban planning... None of that points, in my opinion, towards deep autocratic corruption. If there had been a deep autocratic corrupted regime, politicians would have had higher wages than professors of universities, artists or researchers, wealth inequality wouldn't have been the historical lowest in the region and the lowest in the world. So, what do you mean by autocratic corruption?

We could achieve, incrementally, the same results as through toppling and rebuilding society

That's not how historically systems have changed. The owning class will not let you remove exploitation by voting, in as much as the kings and queens of old went through the guillotine to remove them from power. When you get massive leftist voter turnout, you either get an Allende situation (I'm Spanish, we had a similar thing during the Second Spanish Republic in 1936), or a Syriza situation, historically. You can also check the case of Mosaddeq in Iran for that matter, the list is endless. The western global empire won't allow a peaceful, democratic transition to socialism.