I don't see it. Somebody needed a MIR plugin, now we have them and they are unstable. Worrying about them becoming "de-facto stabilized" just because people use it defeats the point of unstable and goes against "progress without stagnation".
Those using unstable features are martyrs willing to have their crates broken for the sake of a better Rust. The compiler devs should have peace of mind in breaking unstable code whenever they see fit. Those using MIR plugins are this kind of martyr, thanks to them we will have a solid plugin story in the future.
If worrying about breaking unstable code is truly a thing it seems to me that the real issue is that unstable is not unstable enough and what we really need is really_unstable_dont_use_this_or_puppys_will_die.