The way I see it, it can be made part of other features. Have names be auto-reserved locally between instances for future user migrations if they desire. Having the same username for different instances for different people is deceptive as fuck and will be abused as well if it hasn't been yet.
That a user acting in bad faith retains the same name throughout different instances is merely coincidental - I doubt they won't mind switching it up, specially if they are automating account creation and astrosurfing.