I think it'll be EU-style privacy regulations that make it illegal to train on the majority of data. Perhaps the requirement to be able to remove a user's impac

I think it'll be EU-style privacy regulations that make it illegal to train on the majority of data. Perhaps the requirement to be able to remove a user's impact from an already computed model if they file a right-to-be-forgotten. Something that would make any non-trivial model a legal nightmare.