I'd be curious to hear a proposal on how Apple could implement an E2E encrypted messaging service in a way that it would be impossible for them to circumvent it in the future via a software update. I suspect it's not possible, and therefore, either you accept some trust in Apple and take their model as "good enough" or you don't trust them at all (meaning you don't use iOS or macOS). There are, of course, lots of people that do that. But for most the population of iOS users, I think the iMessage model is very much good enough.