But there's no saying that future versions of stock Android will work on a specific device. Because it's open source, many OEMs just take the OS and don't upstr

But there's no saying that future versions of stock Android will work on a specific device. Because it's open source, many OEMs just take the OS and don't upstream the drivers they write. And because Linux has no driver ABI, they need to be recompiled from source for every new version of the OS (and modified accordingly too, because there's no stable driver API either). That's why project Treble was done.