It's way, way too easy to convert culture. Or rather, it's too cheap and too fast.
Full realism may not really be required but spending a single decade to convert a province is so little that it seems absurd (and therefore immersion-breaking) regardless of whether it represents ruling class or everyone. I'd say 25-50 years would be far more reasonable even if still quite a bit too fast for realism.
There is also the issue that very few nations even attempted to do this in real life. Sure this game is about changing history, but maybe there is a reason they didn't do it back then. Maybe it was, I don't know.. DIFFICULT and COSTLY? In my opinion, changing province culture should be sufficiently expensive that it'd almost never be worth it. It'd be an option but not something you'd do often. It should be rare, mostly done out of principle or to switch an especially strategically important province.