So? There does not need to be a way to get rich quick. I don't see a problem with forcing chat providers to include proper end to end encryption as a matter of consumer protection. Because we all know that otherwise, security will not be part of the minimum viable product, and the consumer can't tell the difference.
(Obviously, the state of the art answer to your technical questions is the double ratchet algorithm.)
(Obviously, the state of the art answer to your technical questions is the double ratchet algorithm.)