> 1. it's slow, requires insane amount of storage, power and traffic

Compared to the infrastructure required to uniquely identify every human in order to stop them from registering lots of troll accounts, and implementing a fair and accountable law enforcement apparatus to make sure the rules are followed? The blockchain might be cheaper.

> 2. nobody wants non-transferable username for real

Which is the worse outcome -- you can't transfer your username (but you can register a new one for a fee), or trolls and squatters can trash the system?