What's unrealistic is that Steven and the crystal gems were able to remove the diamonds to power largely without violence and with a very small group. Yes, there was the epic cool battle scene in the finale, but crucially what causes White Diamond to abandon her world view is getting owned with facts and logic(tm) demonstrating her worldview to be irrevocably false. This resolution requires White Diamond to bow to the material reality around her rather than double down, can we realistically see our capitalist oppressors doing the same? And then, even if the diamonds gave up power willingly, those who had been privileged under that system would be unlikely to ALL do so as well (credit where credit is due, they explored this in Future). I guess the point I'm trying to make is that the show doesn't make a depiction of a realistic revolutionary struggle, which they seemed to be trying to do to some degree at least.

As for giving the diamonds the puyi treatment, this actually has some material basis justification in the show. Specifically, they are needed to heal the corrupted gems, end up providing some useful services, and even start to repair all the gems they had murdered (slight cop out if you ask me).