It is probably entirely necessary to do it in this particular situation, but when people argue for no software ever breaking backward compatibility, they're arguing for more and more convoluted solutions like this. There's always a cost.
Toolchains are poster children for that genre, yeah. The ecosystems are too large and too interconnected to evolve in parallel. I mean, any one of LLVM, Rust and Clang are too large as a single project to implement a breaking change like this as a single event, trying to get them to do it in tandem just isn't going to happen. And even then there are uncounted other LLVM downstreams to worry about, all of whom need a story for how to migrate.
I just want to get the people who scream on HN about how software should never ever break backcompat under any circumstance, in a room with the people on this issue who are stunned by how convoluted this situation is, and get them to fight it out. Bonus points for when those people are the same person on different days.