But this is also starting to not be true. Gmail is very slowly adding features not in smtp/imap and one day they might just break away and have their closed ema

But this is also starting to not be true. Gmail is very slowly adding features not in smtp/imap and one day they might just break away and have their closed email. This happened on multiple occasions with xmpp. At one point you could write from icq to hangouts to facebook messanger. After companies gathered critical mass of users they turned off federation. This is one of the main reasons people use facebook messanger (almost everyone has it) and this might happen to gmail quite easily. Almost everyone had gmail at some point so this idea is pretty possible. Many people would then keep gmail account just to comunicate with people who dont have email.