Not sure who the "we" is that you refer to, but Google (and Samsung, and other Android manufacturers, as well as companies building other Linux-based embedded/IoT devices) could band together and create a "Corporate Embedded Linux Consortium", and pool some money together to pay developers to maintain old kernel versions.
If the mainline kernel devs are uncomfortable allowing those to be official kernel.org releases, that's fine: the CELC can host the new versions themselves and call the versions something like "5.10.95-celc" or whatever.
I don't get why this is so difficult for people to grasp: if you want long-term maintenance of something, then pay people to maintain it long-term. It's a frighteningly simple concept.
But yes, it'd be better for SoC vendors to track upstream more closely, and actually release updates for newer kernel versions, instead of the usual practice of locking each chip to whatever already-old kernel they choose from the start. Or, the golden ideal: SoC vendors should upstream their changes. But fat chance of that happening any time soon.
> (there's actually been quite a bit of progress here in getting them to actually take updates from the stable kernel at all! And that's getting thrown away now...)
I found this statement kinda funny. If the original situation was that they wouldn't take updates from the stable kernels, then what were all those unpaid developers even maintaining them for? It's bad enough that it's (for most people) unrewarding work that they weren't getting paid for... but then few people were actually making use of it? Ouch. No wonder they're giving up, regardless of any progress made with the SoC vendors.