I think (and I'm a security know-nothing, so could very well be off in the weeds), the firmware should accept updates signed with two keys. The manufacturer key, which can allow automatic updates, and a post-service key that cannot be automatic. Either a user has to initiate the firmware update manually, or consent via some other means.
This post-service firmware may very well enable a third key for automatic updates of its own, so there's just a manual step on the transition from manufacturer to some community project you support, not each revision afterwards.
Ideally, the key would also be pre-registered somewhere to ensure they can't skip out on releasing it, but I'm not sure how you do that without it potentially leaking before the device reaches end of support. I guess a code sitting in a lawyer's vault somewhere. Again, nobody is paying the lawyer to refresh his vault's firmware image after each patch, so 2nd key wins over unlock firmware again.
Why a lawyer's vault? I know lots of people would love to take ownership of the device immediately, but from the device creator's perspective, they tend not to like that... especially if a device is a source of subscription revenue. So I'm not sure how you'd get vendor buy in to early release unless it becomes mandatory... which I can't see.
Of course, manufacturers don't want that either because they make money off of planned obsolescence and consumers keeping old hardware running makes them less likely to buy old hardware.