Alerting the user when a MITM certificate is active in the trust store is relying on a completely different threat model than "protect the entire operating system against state-mandated malware". I'm saying browsers should at least do the former. You seem to think that's pointless unless they also do the latter, but of course they can't do that. Some security of the trust store is better than no security.
"it rather involves being already at the other end of this airtight d doorway"
the current page ask the user to run an installer, elevating privilege. there's nothing a browser can really do against that. DLL can be replaced and signatures can be tempered etc.
just because you said "ship them with the browse" doesn't make you magically right nor safe under the linked threat