How is the blockchain verified after the last bitcoin is mined?

There would be nothing to mine, it would only be verify transactions. Would the motivation be for large holders to verify bitcoin so they remain in possession of an extremely valuable asset? What stops someone from taking control of the verification process once smaller parties have no incentive to mine (51% attack?)?

This wouldn’t take place until 2140 but still it’s one of the few things that troubles me about the ideals of bitcoin