By compelling one of the endpoints to cooperate - this type of law right here. The first will be bringing non-E2E centralized web services into the fold, roughly sorted by size/prominence and whatever bleeds. Then they will move towards pressuring ever smaller "services" to give up plaintext. Simultaneously the push to censor E2E software will have also progressed (made feasible by centralized app stores). Then, assuming that panopticon for the corporate-mobile-first 90% of the population isn't enough (and it's never enough), they'll move on to criminalizing publishing E2E apps directly, since having long censored the app stores, those will be only something weirdo criminals use.
