Note that it appears there is a 100 registration limit per Google account. And each factory reset changes the ID the phone is registered under. For a lot of ROM folks, this is probably not going to last very long.
Wow, if that's true, that changes the whole tone of this. Hopefully this is just a temporary oversight, and Google allows the opportunity to delete and reuse these 100 slots.
What is your reason for "five years"? And yes, Android ROM folks kinda shock me with how often they fiddle with their phones, and I also don't have time for that kind of business.
I suspect there'd be bursts of burning multiple in a single day, for what it's worth. People flashing a phone, realizing something isn't working after they set up their registration again, and having to flash again.
Five years because there was a link recently on HN that indicated that was the average length of time that the top-end Android phones get updates. It's the only metric I have seen regarding the useful life of an Android device.