You could argue "but this is well researched so it cannot be a fad" but I think they're focusing on the wrong things. Sure, the send button is 4× faster to find according to their research – but I don't want a huge send button near the keyboard. The send button is the most dangerous button in my email client! I'd like it to be small and require deliberate effort to hit.
(Besides, it doesn't move around – I hope – so I will already know where it is when I compose my email. I'm not shooting down a fighter jet. I don't need to acquire the target quickly.)
On the other hand, this seems to be Google backtracking and saying "Ooops, sorry, our previous recommendation of a UI where all components blend into each other looks sleek but is hard to use" so I guess that's an improvement.