I can't speak towards Apple's or $your_government's trustworthiness, but MTLS wouldn't protect against an attack where Apple collaborates with a data requester.
There are people and orgs out there who (justifiably or not) are paranoid enough that they factor this into their threat model.
This is a bit academic right now, but it's also worth mentioning that in the coming years, as quantum computing becomes more and more practical, snapshots of data encrypted using quantum-unsafe cryptography, or with symmetric keys protected by quantum-unsafe crypto (like most Diffie-Hellman schemes) will be decryptable much more easily. Whether a motivated bad actor has access to the quantum infrastructure needed to do this at scale is another question, though.
