Whenever I moved my friends onto Matrix, all I had to do was hit the "Encrypt room" button for each of our rooms. They did not have to do anything else. Is this ideal? No.
Is it E2E for their other rooms? No.
Is it E2E for any usage of matrix they were going to be doing? Yes.
I also told them to be sure to hit that button if they did start any other rooms. None of them ever did, but they knew that they should if they cared about being secure.
Riot.im also did a good job of letting you know that it was unencrypted unless you hit that button.
Edit: What it boils down to, is that Matrix has been easy to set up E2E for as long as I can remember. It is also trivial to create a matrix account with no tie to your IRL self.