Honestly I am not sure why some of you are going on at the OP like this. This is actually a good solution in that it provides native pixel density while providing 21:9 ratio at a reasonable cost on a 16:9 screen. Yes you will have less being displayed on screen BUT the game will be displaying more information than it would if it was using the entire screen.
Imagine if you have a 28 inch 16:9 monitor and you do this on it, the kind of image you are getting would be similar to what you'd get on a smaller 21:9 monitor like 24-26 inch or something. When you say "OP just get a 21:9 mo monitor", you are implying that if the OP bought a 24 inch 21:9 monitor that displays an image the same size as the one OP is getting right now you'd be ok with it...that...makes...no..goddamn...sense !
Yes the 21:9 monitor will have a higher native resolution but OP doesn't want a higher native resolution (GPU limitation), this is a solution for people who want to game at 21:9 on a 16:9 monitor while having the native pixel density exactly equivalent to 1080P.
This topic is offensive. Black bars in games (not movies) is atrocious. Characters and HUD elements already take up too much space as it is nowadays, I'm not making it smaller.
Except they take less space with this because you have a wider FoV and more is being shown.
Just look at the very pictures OP posted