I don't think slippery-slope is an appropriate argument to make here, or really anywhere.
To answer the UI elements on the bottom, I'm not sure I actually press the Send button at all, instead opting for a keystroke depending on the client. The physical analogue I base my request off is the order I fill out a letter or card--I write my message, then I sign it, and then I seal and address the envelope.
Beside that, what's to keep a hypothetical upside-down mail client from putting all of its control elements on the bottom of its interface?
To answer the UI elements on the bottom, I'm not sure I actually press the Send button at all, instead opting for a keystroke depending on the client. The physical analogue I base my request off is the order I fill out a letter or card--I write my message, then I sign it, and then I seal and address the envelope.
Beside that, what's to keep a hypothetical upside-down mail client from putting all of its control elements on the bottom of its interface?