I honestly don't think that it will ever get big enough for billionaires to even take notice. Even if we get 10% of the total social media traffic, which is a massive number in and of itself, the sharks are going to be fighting over much larger stakes.
If the fediverse does somehow manage to get large enough to attract the attention of the pigs that run the farm, I suspect they'll just clone the (open-source) code, make their own instance running on another protocol, and throw their weight around to get everyone moved over there.
Relying on the judiciary or the legislature is a bad gamble.