The main point raised here in contra-stable-API camp was that catering to closed source software isn't sustainable and in fact not even the kernel's job. It is, however, one of the kernel's declared jobs to support even very dated software.
Also, as for old hardware, why shouldn't one use the 2.2.* / 2.4.* / 2.6.16.* branches? Old hardware can anyway not use the latest features, therefore it doesn't _really_ make sense to apply the newest kernel to it. As long as you port back the security patches, you're well off.
On a more general note, I would say, as long as something is open source it is per definitionem (more) maintainable.