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.
Regarding on mobile, I’m familiar with chat apps that require a UI button press to send, and consider it unnecessary friction. It’s a larger mental leap to have to leave the keyboard.
I find this interesting! I've never considered myself "leaving the keyboard" on the phone as if I were task switching. When I'm done typing I flow directly to the send button and let the keyboard go. The fact that the button is above the keyboard makes no difference to me as long as it stays accessible.