I think you’re right. Once Apple’s no longer releasing Intel versions of the OS, that MAY be the end of the road for subsequent releases to show up on Intel systems. Again, having said that, I’m sure that some group may make it their sworn duty to look at what all the ARM code is doing (in macOS XX?) and attempt to write Intel versions of all that and even release a tidy little updater. But, effectively a non-event for folks that wanted to simply take macOS and run it on a MUCH faster Intel PC that Apple doesn’t offer.
And, I also think it’s possible that, since the code will be coming from Apple (when performing any upgrades), it’s likely that each “version” of macOS actually consists of a lot of different packages specifically meant for each system to be installed to. I know it works this way on iPadOS and iOS (where the upgrade is sized for the specific device receiving it), so I wouldn’t doubt that it works that way on any future laptop/desktop Apple Silicon systems.
Another question, are there even ARM BYO systems out there?
System updates (both point release updates for the "current" macOS release as well as the Security Updates for the two immediately previous releases) will include minor firmware updates for Intel Macs now and those are definitely system-specific. I don't know if the firmware updates are baked into the update itself or if the update kicks off a separate under-the-hood process to fetch the updated firmware for the given Mac model. Whereas for iOS and iPadOS, the update for the specific model (e.g. iPad mini (5th Gen, Cellular)) is what's downloaded and processed as any other device's update wouldn't work.
Major macOS releases always contain a TON of changes under the hood. Getting people to band together to re-write that code for Intel would not be feasible, let alone possible. It's a nice idea. I often fantasize about open source forks of Snow Leopard to make a totally different desktop platform. But it'd be a lot of work that ultimately wouldn't make sense. Especially since macOS, on the whole, is becoming less utilitarian as the years pass. I'm going to get another Intel Mac soon. Then in a few years, maybe an Apple Silicon 13"/14" Pro/Air/MacBook, but I don't know if I have that many more Macs in me past that point. I'm just able to do so much more in Windows without limitations or Apple's arbitrary deprecation of technologies like 32-bit app support (albeit, there's not really a need to support 32-bit ARM macOS apps as there ought to not be any).
As for ARM BYO systems, the Raspberry Pi is a popular one. But, otherwise, I don't know of any others. I think that a lack of hardware selection will be the least of anyone's concerns when it comes to whether or not Hackintoshing is viable on a third party ARM system.