My main Thunderbird toolbar has get mail, write new message and quick filtering. It took me perhaps 10 seconds back when I first installed it on this computer to hide a couple of other buttons like chat that I don't use myself.
The second toolbar when I have a message open has just 5 buttons (reply, reply all, forward, mark as junk, delete) and I use all of them frequently.
I agree with your general point that clean UIs that prioritise more important features tend to be nicer to use, but I think you're dramatically overstating the problem here.
Why can't they just show suggestions when I'm in the compose window and I type the first few letters of the person's name or email?
That's exactly what it does, and has for as long as I can remember.