If they don't do E2E, users will abandon Facebook for platforms that do support it (and already are). And they aren't doing this to commit crimes, but to prevent advertisements from targeting every conversation they've ever had.
Honest question: We all know FB won't give up their targeting data, but if it's done locally on-device and we have access to see & modify it, would you support it as a better business model?
But since this idea has been brought up by a number of ad companies (Google, most notably), I can answer in the more general sense.
I would not support ad targeting done that way. It is a bit less objectionable, but it doesn't really address my objections to the entire practice.