But still, desktop Macs have their place. Yes, most Macs sold these days are laptops because they’re powerful and flexible. Measuring at 4096 by 2304 pixels, that’s 9.4 million pixels, providing a vastly broader work area than a 12-, 13- or even 15-inch laptop screen.
The new 4K iMac isn’t the cheapest Retina Mac you can buy-both the MacBook and the 13-inch Retina MacBook Pro come cheaper-but it offers a much larger screen.
In a time when PC growth has stalled, Apple continues to grow its market share and update Mac hardware in interesting, innovative ways. But it’s clear that Apple views the Mac in general and the iMac in particular as a point of pride. Apple could probably be forgiven if it put the entire iMac line into maintenance mode: after all, sometimes it seems like that’s what most of the PC industry has done. It's just a handy option to have.The Mac is a small fraction of Apple’s total business, and of that, roughly three-quarters of the Macs sold are laptops. The USB-C Prioritization option gives you that flexibility by allowing you to choose whether to have the display maximize USB data speed to run video and USB 3.x simultaneously - which is great when running a single display - or to instead maximize video bandwidth and only runs USB 2.0 speeds, which can be useful if you're more interested in setting up a daisy chain than maximizing USB data speeds.
But if you had a display that allowed you to configure it to set up the USB-C link to maximize video bandwidth, creating a FULL video bandwidth link, then you COULD run a dual QHD 60 Hz daisy chain from a DisplayPort 1.2 system. The catch is that many displays only support setting up the USB-C link to carry USB 3.x, and in that case you simply wouldn't be able to have a dual QHD 60 Hz daisy chain from a DisplayPort 1.2 system. But some people might be fine only having USB 2.0 speeds available for things they plug into their displays (keyboard, mouse, etc.) or might not use those USB ports at all. The downside to that scenario is that you'd be limited to USB 2.0 data speeds. A half video bandwidth link from a DisplayPort 1.2 system is NOT enough bandwidth to run dual QHD 60 Hz. But suppose you wanted to set up a daisy chain. And that's why QHD displays with USB-C inputs and USB 3.x ports configure the USB-C link to support video and USB 3.x simultaneously.
With that type of system, a half bandwidth video link is enough for a single QHD 60 Hz display, which means you can run that plus USB 3.x simultaneously. Most systems on the market today only support DisplayPort 1.2 over USB-C. If you want USB 3.x, then that requires two of those four high speed lanes - but that means that your available display bandwidth gets cut in half. Instead, you can only use USB 2.0, which flows over pins in the USB-C connector dedicated to that purpose. If you dedicate all four lanes to video, then you get a full DisplayPort interface worth of bandwidth - but you can't run USB 3.x. because I haven't seen them side by side, but if you'll be using it for coding and general productivity work, that might not be much of an issue for A USB-C link has four high speed lanes that can be dedicated either to video or USB 3.x (or Thunderbolt, but that isn't relevant here). I can't speak to how closely the image will resemble your iMac's image in terms of color gamut, contrast, brightness, etc.
If you want a USB-C input for futureproofing (and/or so you could use the display's DP input with the iMac and the USB-C input for a laptop), then you might want to consider the U2719DC instead. The current equivalent would be the U2719D. In terms of specific display recommendations, I have a pair of U2717D displays. They both support it, but on both platforms it means that all displays running a scale factor OTHER than the one the OS is optimizing for won't look as good as they would otherwise.
Both Windows and macOS have problems with that. I have 27" 1440p displays myself and I love them, and having consistent pixel density on both of your displays will result in the most consistent experience and the best graphical presentation compared to having to run different scaling settings on different active displays.
There are certainly 25" 1440p displays, but I think I too would find that tough to use. If you have a non-Retina iMac 27", then its internal display is 2560x1440, in which case I would certainly recommend matching both size and resolution for your external display.