What? 27" 2560x1440p is better at having 2 windows side by side than 29" 2560x1080p. Both can be found around the same price, have the same horizontal pixel count, and panel width of a widescreen 1080p 29" is only 13% wider than a 1440p 27", so while being more panoramic, the content displayed is a bit tinier. But with the 1440p panel, you get 360 pixels more in vertical, very helpful in multi window display, and when rotated to a vertical position if the panel allows it to achieve a 1440x2560 resolution, the experience becomes premium at web browsing or document writing.
And btw, multimonitor doesn't count as an argument in a ratio aspect discussion, as there are tons and tons of possible configurations suited for the best use.
Yes, a 16:9 screen of the same width is a superset of a 21:9, in much the same way that a 21:9 screen of the same height is a superset of a 16:9, and I'd argue that the 21:9 of the same height is always a superior monitor, and is superior in a way that a 34% larger 16:9
isn't. That 16:9 would be 15% bigger in each dimension, so roughly 3000x1660. That's got some really tall columns. It'd probably be solid for reading two docs side by side, but if you've got bookmarks on a pdf or something similar that's a source of problems. Same for browsing online with a tab list down the side like I use (tree style tabs). The real killer for me is that when I'm running two halves of a screen I can turn a bit to the half I'm using, while too much vertical range can be a problem. So the near square halves of 21:9 are really practical when using a very wide screen close up like a 34" 3440x1440. There's a certain point where if you want more space you have to go wider rather than taller just because of the way the human neck works.
The cost argument is definitely a problem for 21:9, but the ergonomic qualities of a screen have not, are not, and will never be determined by economies of scale, so making that argument is a cop-out when trying to determine which is a better aspect ratio to work on.
The reason I mentioned multimonitor is because that's what it takes to try and assemble something with the same capability as an ultrawide out of the same height 16:9 screens once you acknowledge there's some point at which going taller is limited.