Whose mandate? Are you going to make a law saying you can't customize Google's base Android?
It's an open source OS, manufacturers offer crappy support because they want customizations and proprietary software but don't want to have to spend a bunch of engineering time to keep pace with Google's reference spec. Samsung does, but that's because they're the literal largest phone manufacturer on the planet.
But Google can't be out there saying you don't get to use Android code if you don't offer timely support for a decade. There's a reason years of security updates are now a declared selling point, the only force to drive it is market pressure. At most you could regulate that you HAVE to support swapping OSs on phones, but you can't just target that at Android and not Apple, and Apple would buy themselves a nuke to fight against that one.