Since mention feature added,we are not able to change our usernames anymore. But I believe this could be fixed by sacrificing mention feature. Here below I explain why this sacrifice will not be a major one,but just a minimum one,and having the ability to change your username is definitely more important.
1) GAG Used To Function Without This Feature For 7 Years.
This pretty much says it all. It was only added 1 year and a few months ago (correct me if i'm wrong,don't recall exact date). And since GAG could survive without mention feature all these years,it's pretty much clear it's not that important.
2) Since When This Feature Started To Exist “Tagging Questions” Have Been Increased.
Got to admit i have asked similar questions many times as well. But there are many users who get bored with them. So for them sacrificing mention feature would be a relief.
3) We Can Still Respond To Other People’s Comments Who Are Not Opinion Owners Without This Feature.
Exactly. We can simply send them a PM and tell them something like "I responded to a comment you left on (enter username here) opinion here in this question (enter link here)". So technically this works as a notification.
4) When We Created This Account We Didn’t Know Wether We Will Stay In GAG Or Not.
Many of us,including myself,joined GAG either in order to just ask a question or in order to respond to a question we found via some search engine. We didn't know that we will stay in GAG for a longer time,so we didn't think much about our usernames and we chose as a username whatever we had in our minds the moment we were creating this account. Why suffer the same name then?
5) Many People Get Bored Seeing The Same Username For Years.
Even if you liked the name you chose the moment you registered here,it doesn't mean you would like having the same name forever. One day you might get bored being called (enter username here) and you would like to change it after some point.
6) Many People Wouldn’t Start A New Account Just For Changing Their Usernames.
Imagine someone having an Uber-moderator status,an Editor status,being a Master and having a fairly high MHO percentage. I doubt he/she would throw everything away just in order to change his/her username if he/she gets bored with it.
7) Most Online Communities Allow You To Change Your Username At Least Once
So why not in GAG as well? It's a little bit unfair don't you think so?
These are the reasons why i strongly support having the ability to change your username,even if you have to sacrifice Mention feature.