2560x1080 is a bigger screen than 1920x1080. It is much more expensive as a result.
Shock horror that a bigger screen with more pixels is better than a smaller screen with fewer.
Unless you keep the pixel budget or the price constant, you are not debating the merits of aspect ratios, you are debating the merits of having a bigger screen.
Which is also why 2560x1440 is a better screen than 2560x1080.
Lastly, it is true that *any* 21:9 screen will show more width than *any* 16:9 screen, due to the way vertical FOV is fixed in 99% of games. However, if the vertical resolution is smaller on the 21:9 screen, then you lose detail. As already demonstrated, if the vertical resolution is the
same on the 21:9 screen, then it is simply a
bigger screen than the 16:9, and hence not a fair comparison.
TL;DR:
1920x1080 (16:9) -> 2560x1080 (21:9) = bigger screen, more expensive, more pixels, same vertical res, bigger horiz res.
2560x1080 (21:9) -> 2560x1440 (16:9) = bigger screen, more expensive, more pixels, bigger vertical res, same horz res.
In both cases you are not focusing on the merits of one
aspect ratio vs another. You are debating the merits of having a bigger screen vs a smaller screen.
Because it doesn't work that way, 21:9 was based on 16:9, not vice versa. The 21:9 equivelent of 2560x1440 is 3440x1440.
Lastly 21:9 is not "based on" 16:9.
That's exactly the same as saying the number 3 is "based on" the number 2.
As an abstract mathematical concept, 21:9 is simply different to 16:9, and one is not derived from the other. Ask any mathematician.