Comment by jorams

The reason the send button on the old design is hard to see seems to me to be that it doesn't stand out in any way. The only difference to everything else on the screen is that it's blue instead of black, but the contrast isn't big and it's between two less important icons.

Here's a 30-second edit of the first picture that undoubtedly breaks material design guidelines, but also solves the problem without introducing any new problems: https://kappa.lol/7Zuuc8.png

The problem with a text button in a case like this is that the translation of "Send" is longer in most languages and even much longer some languages.