That's assuming they make a final firmware update. Having the 2nd manual key available from day 1 ensures the device is unlockable with just a release of the key. Having a 2nd key or a signed unlock firmware update I guess are two ways to achieve the same goal, but the 2nd key would be better. The 2nd key likely stay in place forever, in each update, while the unlock firmware would likely end up remaining as the original firmware, because why would most vendors build two firmwares for each release. It would sit forgotten on a drive somewhere. The use of original unlock firmware could mean making a device vulnerable between loading the unlock firmware and the community firmware, so the 2nd key is preferred. It's always ready.

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.