it's been like a million years since I used the default non-legacy gmail UI but now apparently I don't have any choice; anyway I noticed it gives you a big old warning banner when you try to email someone from a different domain from yours?
neat ... neat mail client guys
anyway I have no doubt that google could unilaterally decide to kill email if they decided they wanted to; everyone would hate them for it, but everyone already hates them for killing Reader, so what have they got to lose