I’m more forgiving of that kind of thing with older games that predate ultrawide resolutions, and consider it a pleasant surprise when I find an older game that works fine with it. But since I’m running a Surround setup, I have the ability to just turn off a couple of monitors and run in 16:9 if I have to - which I do for most older games. It really sucks there isn’t a good workaround for you, and others with 21:9 screens.
But it’s bloody annoying when it’s a new game that doesn’t support anything but 16:9, or only supports it badly. The only argument I can see against supporting wider resolutions is that in competitive games, apparently the wider field of view offered by screen resolutions wider than 16:9 offers an unfair competitive advantage to the players that have them. (Like one person having a better CPU or GPU, or more RAM than someone else doesn’t?!) With single player or cooperative games, where there is no competitive element that gives an advantage to whoever has the best hardware, I really can’t see any justification for not supporting non-16:9 resolutions.