No one uses the UI button to send a message on the desktop though. Everyone just presses Return to send, which is the most common need, and then once in a while realise they need to enter a newline without sending, for which there isn't a button so they need to learn how.

Mobile doesn't necessarily have this issue because it can show the send button and the newline button at the same time and they're equally accessible.

Regarding your edit:

> they would consider it too much friction to have to tap a button above the keyboard for every send

My finger travels almost the same distance from the home row to hit the send button above the upper right corner of the keyboard as the newline button on the lower right. I've been doing this for ages.