Second i bet you that most people find the send button easier when it's somewhere they expect. Where the other buttons are.
Third, physical analog bla bla, really... Nobody thinks "hey i am writing an e-mail, that's just like real mail, EXCEPT i write on a keyboard, look on a screen and everything else i touch and see is not a physical letter or pen or paper at all!". The real-world vs virtual-world comparison makes most sense for icons and such, in my oppinion. Not so much for workflows. You also don't ask people to walk to a virtual post box to send the letter. Or let them "stamp" it. Or as you said yourself, to sign it or to seal it.
Nothing keeps one from putting ALL controls to the bottom, a good interface may even accomplish that nicely. Except that it's only there to annoy people, restrain adoption and giving users a hard time figuring that out. You may be able to quickly learn that, i may be able to do that. My mother or grandmother? Not so much.
It'd be very interesting to see some A/B testing on this. Maybe i'm wrong, who knows.