Something else to consider: Once the revival technology works, you're not going to revive all the frozen people at once. You're only going to revive those whose underlying medical problem can be fixed. And the frozen heads pretty much have to wait for upload tech.

It is true that if successful cryonics will function on a roughly first-in, last out arrangement (since the preservation technology improves over time). And yes, you aren't going to be able to revive everyone at once. However, the head only preservation does not need to wait for uploads. If we have the tech level to revive people one may be able to construct robot or cloned bodies to attach the head to. Moreover, since heads are smaller than full bodies one can more carefully reduce the temperature and make sure that the cryopreservants have infused everywhere. In theory, this should in some respects make there be less brain damage to head only preservation. If so, neuros may be revived early.