I'm afraid this will end up the same way as Treble, and other similar efforts. On paper, this is a fantastic idea. In practice, most vendors will probably block these, or change the core distribution to the point where these are irrelevant, like Samsung or any Chinese market phones do. Not to mention that modifying anything on your device is becoming exceedingly difficult, depending on which vendor it's from
Treble is pretty good IMO, I'm using a device with LineageOS GSI, and it works pretty much perfectly. I do wish it was officially supported by LineageOS, as updating requires a PC right now, but it is pretty usable and a better alternative than using the Stock ROM.
Last I checked, TWRP couldn't flash logical partitions that make up where the GSI is patched, but it was requested, maybe something changed? Idk, the guy who created the guide on how to flash GSI on my device's XDA forum mentioned this issue a while ago and recommended flashing through ADB.
Actually, I think the issue is that when I flashed my device, TWRP couldn't unencrypt or something, it's been a while and I know I should start the process from scratch
Since they are adding it as a part of Google Play Services and Google certifies Google Play compatible devices, hopefully this will be a requirement if they want to include Google's playstore.
A lot of it is actually already required. I can't find any comprehensive list, but each new release since Android 10 (when modular components / Mainline became a thing) adds new components and makes some older ones mandatory for Google Play devices.
Whaddya mean the same way as Treble? Treble works and is used on Samsung or any other Google Play enabled device. If a phone shipped with Google Play, you're getting app updates via Google Play, then you're also getting Treble system updates.