Have you used a 4K display, for at least a few days? If you have, I still disagree with you, but if not, I’m going to completely ignore your opinion, because I find the difference in how pleasant it is to use a good screen just so vast. Sure, you can do things on a lousy monitor, but it’s terrible and you’ll hate it. :)
(My first laptop was a second-hand HP 6710b at 1680×1050 for 15″, and that set me to never accepting 1366×768. So my next laptop was 15″ 1920×1080, and now I use a 13″ (though I’d rather have had 15″) 3000×2000 Surface Book, and it’s great. Not everyone will be able to justify the expense of 4K or similar (though I do think that anyone that’s getting their living by it should very strongly consider it worthwhile), but I honestly believe that it would be better if laptop makers all agreed to manufacture no more 15″ laptops with 1366×768 displays, and take 1920×1080 as a minimum acceptable quality. As it is, some people understandably want a cheap laptop and although the 1920×1080 panel is not much dearer than the 1366×768 panel, you commonly just can’t buy properly cheap laptops with 1920×1080 panels.)
I'll go one step further: I used the LG 27" 5K Display for two whole years before returning to a 34" Ultrawide with a more typical DPI.
Obviously I preferred the pixel density and image quality of the high-DPI screen, but I find myself more productive on the 34" Ultrawide with a regular DPI. (FWIW, LG now has a newer pseudo-5K Ultrawide that strikes a balance between the two).
I look forward to the day that monitors of all sizes are available with high DPI, but I don't consider it a must-have upgrade just yet.
Also Note that Apple made font rendering worse starting in OS X 10.14 by disabling subpixel AA. Using a regular DPI monitor on Windows or Linux is a better experience than using the same monitor on OS X right now. If you're only comparing monitors based on OSX, you're not getting the full story.
I also have 3440x1440 on my Dell monitor at home, and I love it.
My work monitor is a really nice 27" 4k LG monitor, which a coworker picked out. He's a real monitor specs nerd and made a lot of assertions like the OP. The scaling issues are endless and really bother me, and I don't notice the higher PPI at all. I much prefer the ultrawide Dell - it gives me a feeling that I don't even need to maximize my windows and I can still have lots of space.
It takes up zero desk space (one monitor arm that lets me adjust it anytime I want), and I don't need to rotate it. I've never found that a useful thing to do.
On the other hand, when you're done work its amazing for flight simulator or other games that support the aspect ratio properly.
Not parent poster but, as somebody who works on macOS, my problem is that the switch between the high-density laptop screen and a “pixelated” external one is extremely jarring. At one point I had an additional, average Dell monitor and every time I moved my eyes I cringed. After a couple of days I just removed it - better to have fewer screens than forcing my eyes to readjust every few minutes.
So yeah, if your primary device is a modern Mac, you really want a high-res, high-density screen.
Yes, Apple never completely embraced the somewhat hacky techniques of font rendering (hinting and subpixel AA), thus their fonts always appeared less crisp.
Not really.
There is an end point.
Also, resolution is not free, this is a tradeoff, when you push more pixels on screen it means more work for the GPU (or CPU in some cases) and more loading time and space to load all those high-dpi resources...
At this point I clearly prefer lower latency and higher framerate over more pixels.
Sure, to a point. But the step-up in question here (1080p to 4K, at sizes like 15–27″) has clearly visible benefits.
And sure, resolution isn’t free, but at these levels that was an argument for the hardware of eight years ago, not the hardware of today. All modern hardware can cope with at least one 4K display with perfect equanimity. Excluding games (which you can continue to run at the lower resolutions if necessary), almost no software will be measurably affected in latency or frame rate by being bumped from 1080p to 4K. Graphics memory requirements will be increased to as much as 4×, but that’s typically not a problem.
That's true, undeniably. But I think the better tradeoff still is to go up in size: IMO the optimal monitor size is 38". Big enough, but not too much head turning. Would I get a sharper 38" if possible? Sure. But I wouldn't compromise on size to gain higher DPI.
I have a laptop with a 13" 3200x1800 monitor; not quite 4k, but higher resolution than my eyes' ability to discern fine detail. My other laptop is 1366x768. The other laptop is a lot better for a wide variety of reasons (and also an order of magnitude more expensive) but the display resolution is genuinely something I don't give a crap about. It isn't terrible and I don't hate it. There's plenty of stuff I don't like about the display; narrow viewing angle, poor contrast, prone to glare- but the resolution is fine.
(My first laptop was a second-hand HP 6710b at 1680×1050 for 15″, and that set me to never accepting 1366×768. So my next laptop was 15″ 1920×1080, and now I use a 13″ (though I’d rather have had 15″) 3000×2000 Surface Book, and it’s great. Not everyone will be able to justify the expense of 4K or similar (though I do think that anyone that’s getting their living by it should very strongly consider it worthwhile), but I honestly believe that it would be better if laptop makers all agreed to manufacture no more 15″ laptops with 1366×768 displays, and take 1920×1080 as a minimum acceptable quality. As it is, some people understandably want a cheap laptop and although the 1920×1080 panel is not much dearer than the 1366×768 panel, you commonly just can’t buy properly cheap laptops with 1920×1080 panels.)