why are usernames permanent?

[The Tadpole]

The Battle.net system does do this number system, they call it a BattleTag. The BattleTag is permanent, while you can change your display name as you wish.

Normally systems don't allow it, not just because then they'd have to build their security and banning system around numbers (which isn't that big of a deal really), but because names are just easier to remember than numbers in general. Even if you did have a publicly viewable number, if you went and trolled a person, then changed your name, chances are that the person wouldn't recognise your number even if you went back the day after.