I regularly use a Dell P2421 and a 5K iMac 27" display (218 ppi) at their default resolution/scaling.
Native 1920x1200 for the Dell (as scaling makes no sense at these low ppi imho) and 'looks like 2560x1440' for the iMac screen (2x scaling).
On the Dell text is obviously more 'rough'/pixelated whereas on the 'Retina' 5K it looks smooth like printed on paper (similar to an iPhone/iPad).
Another benefit of higher resolution displays is the ability to zoom out further (a.e. in Excel) and still be able to read small text/numbers that would be impossible to identify on your current display.
But the biggest advantage I see from a productivity standpoint is the real estate you gain with a 27" display (or a HiDPI one at scaled resolution).
You could consider looking at some
4K (24" or 27") displays which cost a fraction of 5K and might offer a similar improvement over your current Dell (sharper and more real estate at a scaled resolution).
Numerous users are very happy with a 27" 4K display using the 'looks like 1440p' scaled resolution.
(There are only few use cases where the uneven scaling factor is hindering. Like pixel based design work.)
Not sure this helps with your original question.
EDIT: Don't get me wrong - one can be very well productive on a 24" 1920x1200 display. The Dell is not bad at all and I love the 16:10 aspect ratio.
HiDPI is just more pleasing and more real estate can usually improve your workflow (less scrolling, more items visible, …)