Graphene OS only supports devices for as long as the manufacturer is providing security updates for the phone's firmware. Firmware is binary blob, so there'd be no practical way for anyone else to provide/develop security updates once the manufacturer is no longer providing official updates.

Their partnership with Motorola, I think, involves some ability of Graphene OS devs to access/harden/update the firmware, but I'm not 100% sure. Firmware on phones, especially for the baseband processor, often involves a nasty confluence of copyright, trade secrets, patents, and government rules/demands.

It can be done, fairphone rather famously did it once.

But it is vastly uneconomical, and I doubt anyone is going to start doing it regularly.

We really need some kind of regulation demanding firmware support for longer. The EU seems the most likely entity to achieve something like that. Phone vendors can't even control how long they support their own hardware, because the SoC is almost always Qualcomm, and once they drop support, there aren't any good options left.

Ah righteo. Thanks for updating my understanding.