> Aren't they mainlining everything so it gets supported "for ever" by Linux?
No, they're not doing that at all. They're still going to depend on the SoC vendor for firmware and driver updates. Upstreaming a tiny portion of the support for the hardware doesn't mean the hardware gets supported forever. There also aren't some automatic group of people who properly maintain drivers upstream. Upstream kernel drivers regularly break and heavily regress due to lack of use/testing.