I don't think it's a case of Disney allowed anything to happen. It's just a case of it took Universal a long time to catch up. I don't think Disney was ever reserving the space of intricate theme parks all to themselves through any action of their own, it was just that the other parks either weren't trying to do that or it took them a long time to figure out how to do it their way.
Theme park fans, especially Disney fans, seem to think that Disney has all this time been this monolith that keeps everyone iced out so that they and they alone can stand on top. But that's never been the case. Disney has, going all the way back to Walt himself, drawn inspiration from other places and been happy to welcome new players into the world they operate in. Disney's never tried to stop Universal from growing or coming up to their level, and there's no reason why they should either.
It's like everyone wants to watch these two resorts fight each other when the reality is that neither of them has ever or will ever be interested in doing that.
I genuinely believe Iger means it when he says he views Epic as a good thing for Disney. And frankly, for once, I think he's right. Everybody benefits from Epic Universe for multiple reasons.