Why would SRI protect against a MITM, who can simply change the embedded hashes? (not to mention that there's plenty of non-"resource" content that would not be hashed in the first place, yet an attacker might want to change)

Unless my understanding of SRI is totally wrong, it is to protect against malicious changes to third-party resources, usually by the hosts of those resources (or someone attacking them) or a poisoned cache thereof, not to guard against MITM.