Egos and personalities. , even the Beatles fought with each others. Athletes need minutes and numbers to get than next big contract. The ones not getting paid as much will think they can go be max-contract players somewhere else, AND get the respect they deserve.
Health. They aren't going to be lucky forever. Durant wasn't this season, but the timing was just early enough to let him come back for the playoffs. Curry is always on the edge of an injury, and the stops and cuts he makes will catch up, just like they did with Manu a lot of years. And some of the other guys who haven't had injury problems? Sooner or later.
Plus the fact that they can't play 5 guys for 48 minutes per game. They'll need role players. A lot of ring-chasers are guys who were stars somewhere else, but they're aging. A lot of them come in trying to prove that they can still be starters, and the ego problems get worse. A lot of them turn out to not be worth a damn. A lot just don't fit in, and the chemistry suffers.
The Warriors are somewhere near their peak right now. How long they can keep it up depends on a LOT more than Bird rights.
Keeping a team together is a lot harder than most people think. Especially one built around cheap rookie contracts, once they begin to mature. We saw it in OKC. Portland had a powerhouse stable of young talent, but they were still building when those rookie deals started expiring one after the other. One bad draft pick (Oden) and one bad injury (Roy), and they were done even before they got started.
Even most Spurs fans don't appreciate what the Spurs did here for 20 years.