They would have to find someone else if they grew too big.
Though, the secondary doesn't necessarily have to be a maintainer or even a contributor on the project. It just needs to be someone else to do a sanity check, to make sure it is an actual release.
Heck, I would even say that as the project grows in popularity, the amount of people required to approve a release should go up.
So if I'm developing something I want to use and the community finds it useful but I take no contributions and no feature requests I should have to find another person to deal with?
How do I even know who to trust, and what prevents two people from conspiring together with a long con? Sounds great on the surface but I'm not sure you've thought it through.