Thanks for the clear explanation. Even I mostly understand it!
Is this going to cause upgrades and rebuilds to fail on sites with theme components that don’t play by these rules? As happened to Failed to Bootstrap, due to discourse-alt-logo theme component for another reason?
If so, is there a way to make the error message more clear (to people who can’t make sense of the build logs)? And say something like “remove x plugin before trying to upgrade”. Or, better, is there a way to warn people on the admin panel when we know such a change is coming? (or maybe there already is one that no one reads).