You probably will sit half a meter (1.64ft) from a 26.5" monitor, maybe more, right?
According to isthisretina.com the density of a 26.5" 1920x1920 becomes equivalent to retina (326ppi) at about 34 inches (86cm).
Typically I have three displays in a row: a monitor on the left and two computers on the right, a MacBook Pro Retina and a ThinkPad W520 (145 ppi). Whichever computer I'm primarily working on that day goes in the middle and is connected to the external display.
I wouldn't be able to use a display 34" from my eyes unless I used a different pair of glasses, and then I'd have to switch glasses back and forth to look at the different screens! Unless I gave up using the laptop screens at all, but I'm not about to do that.
So with all my monitors at the same distance, what feels like a "retina" density is the same on all of them.
I was just stressing the fact that depending on the size and distance, even sub-100ppi screen densities might be good enough.
Admittedly if you sit 20" from the screen regardless of its size you may want a slightly higher ppi.
If someone can easily focus at different distances, then of course that opens up more options.