Many of us don't do any work on our gaming PCs or not serious enough work that desktop real estate is an issue. And 16:9 4K is the future anyways. I am sure a 4K 40" TV wipes the floor with any ultrawide in productivity and doesn't actually cost more either. They are just slow for gaming currently which is a problem that will be resolved in the future.
Personally, I find 16:9 to look rather dated when compared to 21:9. That didn't stop me from getting the Acer XB280HK as my primary gaming display which looks pretty good imo. 4k is a much better gaming experience when it comes to image quality and raw detail; but we don't have any 21:9 4k panels as of yet. It adds about 3 million more pixels which is a considerable increase.
A 21:9 2160p panel would be preferable. 16:9 is a rather low field of view in gaming and simply lacks the real estate that 21:9 provides.
The 21:9 price tax is far too much for me to take it seriously at the moment. I know many who love their 2560x1080 displays but that is too low of a resolution for me. 4k is a stepping stone and is a large step forward in the right direction.
I keep talking about 8k120hz, but now I may be willing to settle for 21:9 2160p (5040x2160). For 4k, Aliasing is clearly visible at optimal viewing distance and AA is far too much of a tax for my 1070. Going anywhere lower than 4k at this point makes everything look fuzzy unless the PPI is high. No amount of anti aliasing will make 1080p or even 1440/1600p look as crisp unless the PPI is really high (smartphone display). Going to 8k will make 4k look dated as well...
8k is the resolution for monitors, 4k is the resolution for laptops. I hope we have a plethora of choices for aspect ratios soon. VR may simply be a better alternative at that point for gaming. I think the best option is to (one day) get a 4k 21:9 120hz display (5040x2160) as every game worth buying will support it. 21:9 is an improvement over 16:9. It looks silly at first, but so did 16:9 laptops.
21:9 isn't the best either, but it makes 16:9 look cramped. Our eyes can percieve about 200 degrees horizontal and 100 degrees vertical. We simply need a more immersive experience for gaming and applications... Something like a virtual reality... hmm...
edit: For reference, you can comfortably play at 90 FoV on 16:9. On 21:9 you can go to 110 comfortably and some may prefer 120. So much more of the game is shown with the increased FoV, giving many who play on a 21:9 an advantage.