One thing preventing unlocked packages getting stuck in testing would be removal from testing if the maintainer hasn't promoted it in a month or something. Should be enough time to react.

This would also be a way to clean the queue now unless there are clear cases of packages that should be promoted. Anyway, I'd put the packages that have been in the queue and unlocked for a long time under a microscope and reject them even for small issues since most likely those packages will not receive fixes anymore.