As someone who has very crude high level understanding of encryption, I think this post has opened my eyes, I always used to think that End-to-end Secure email is possible, just not widely available right now due to lack of support, and they will get picked up as with wide company support like when Gmail will adopt them. Now I know that it is futile to build them, May be some email alternative from Signal or Matrix will be better. All points made by OP makes total sense.