Sure, I get that. I get there is a whole weird and wacky world of email use that is considered legitimate and needs to work that way for a myriad of reasons. I don't get why their MTA cannot at least have an option to reject mail from your domains if it's not being sent using your account credentials.
The average layperson will not get that. I'm fairly sure if my mother received an email that wasn't delivered to a their spam folder saying "Hey, remember that old copy of my birth certificate you have floating around? Could you send that. Also, CC my good friend bad_user@fastmail.com" that she would call me first - if I was reachable. Also is totally ignorant of digital signatures and most likely unable to verify any present anyway.
As much as I dislike Google and try to avoid their products and services at all cost, at least I have confidence this wouldn't happen with them. Not that I would go back, but it's still concerning.