Actually, there are five princesses. You forgot Flurry Heart, and I think she’s only one to be born into the title.

Bronies aren’t a uniform entity. Back when the “new pony” you’re referring to was in the show, the vast majority of people in the fandom were explicitly against making anything NSFW with them. I’m sure some random people didn’t care and did it, anyway, but they definitely don’t represent the fandom as a whole.

The creator left after the first season, but other than that, I don’t think there have been any major exits other than single writers moving on by choice over time. (And even the creator chose to leave, too - she wasn’t forced out.)