It is next to impossible to change to a new username for two reasons:

1. The majority of user accounts are people that view only and don't want to stream. There are 140 million active users and only 7 million streamers!!!
2. It takes forever for an inactive or closed account to release the username, much longer than the "advertised" 6 months, and it's been already 8 years since you said you'll do something about it.

A good idea would be to use aliases/discretionary usernames for existing and new users that are not affiliate/partner (active streamers), like discord with the #xxxx. Once a user reaches affiliate then they can choose a username of their own.