Nothing. But there's little incentive to do so, at least in my experience 2.5 years into going source-available. Commercial open source has an incentive to rug-pull to source-available (or closed source) because open source is incompatible with most sound business models, but fair source is already source-available and is quite compatible with good business. Of course, O'Saasy has that for it as well, but it, like ELv2/others, tethers the viability of the product to the success of the original author. If the author goes, so does the product. This is unavoidable simply because most users won't self-host as an alternative to a vendor closing shop, and nobody can ever host it for them without DOSP. Rather, they'll go find another hosted product, and the product that promised longevity will die.
But not a lot of vendors think past themselves, which is why backwards user/customer relationships like that of open core exists, where the customers get the shaft when something bad happens to the vendor, but the users are free to continue doing what they've been doing.
For some vendors, that's by-design. If they fail, they want the product to fail too. That's totally fine, but not for me, especially when longevity is promised to users and to customers alike.
Sorry that got a little ranty, but I don't feel like shortening it.