The whole idea of separate long-term maintained software releases is, in my usual most humble opinion, quite broken. (The technical term would be "fubar".)
A fear of regressions is of course real, but better addressed by test-driven development, code review, better quality assurance, and other such mechanisms.
The fear of depreciated functionality (ABIs, APIs, drivers) is, of course, real too: but the solution "backport selected fixes" has got it exactly backwards. Instead, the functionality should be *forward-ported* for the duration.
External large patchsets that mess with internals are another problem; that is not readily fixed, but I doubt that backporting fixes them much better. They're not community players, and the community doesn't really benefit from them. So it makes sense to off-load their cost back to them; let them maintain their own backports. But be willing to help them with forward-porting, merging, or even give reasonable consideration to them maintaining an API/ABI they can rely on.
The (real or imagined) need for a "stable" base very much indicates a failure of the development process to me. It is time to fix that, instead of perpetuating the problem.
(And I'd be grateful if instead of responding to implementation details that I am missing here and which I assure you I am quite acutely aware of since this post would otherwise be not so succinct, critics would instead focus on the strategy, not the tactics. Thanks. ;-)