Why is `RUSTC_BOOTSTRAP` so actively discouraged?

Thanks. I've thought about that RFC a lot, since you posted, and have finally organised my thoughts well enough to add a comment. Overall, I'm conflicted on this topic, as I see the danger to stability-as-a-feature from having any experimental feature usage on stable. At the same time, I see, and feel, the clear need to have a more stable release channel where opt-in is possible. At least now, my original question is well & truly answered.