Earlier quoted context omitted.
E-mail is far from being perfect (you can get you account unilaterally closed by your provider or you can lose your domain name), but in practice I've been using the same address for more than a decade, and I have aliases that are meant to last forever (my almuni address), while in the same period I've had 5 different mobile numbers that I used for services like banking or IM, which is very inconvenient indeed.
I've had the same mobile number for the last fifteen years too. In NZ at least (not sure about other countries) it's trivial to take your number with you when you get a new SIM from any mobile provider. Though this wouldn't work across countries obviously.
Also in some countries keeping your number isn't cheap. And even when it's technically easy, your phone plan might be provided by your employer, in this case it might be very tricky to get your number migrated when you change jobs.