Why is `RUSTC_BOOTSTRAP` so actively discouraged?

automation can help a lot with this. My standard setup is to have dependabot run daily with auto-approval on all PRs where nothing breaks. This only works because I have meaningful, complete test coverage (that's how I code out of preference). Still, if a downstream dependency makes a breaking change I have to go fix it (one crate in particular does this often for something I care about) and yes, that's often a hassle and very easy to overlook for longer than I feel comfortable with. But at least I can rely on cargo to ensure no-one else gets broken, "just" held back.