I just wanted to explain that people easily get used to things they always do, like tapping the send button twice for every message in a hurry, and then find out that they missed something important to add or to correct, that is why this implementation would fail its mission in practice.
I welcome your idea to enlarge the text field and (maybe?) lay it over the rest of the screen while writing the message. I have heard that the devs are collecting ideas for an updated UI/UX, so your suggestion might suit pretty well to this project, and I am sure your voice will be heard among many others…
Edit: There is something wrong with quotes in here, I apologize…