I have 1x 43" 4K monitor (Dell P4317Q), and together with a tiling WM it's pretty awesome as you say. It's more expensive (I think $800), not sure if it's sufficiently better as to be worth it.
I have the same (single) monitor and love it. $699 at MicroCenter.
I like to sum up this debate like this:
With multiple monitors, you're confined into smaller, "hard-coded" boxes that can't be changed. With a single large screen you have much more flexibility.
You need extra functions in your window manager for moving between screens with multiple monitors. Depending on their placement the shortcuts might be non-intuitive. Different screen resolutions lead to unnatural movement of the mouse cursor between screens.
I've also found that such extra complexity impacts productivity negatively. It's an unnecessary distraction trying to communicate such complicated layouts.
I'm just using a 27" 5K iMac, BUT sometimes I change its resolution from the default, virtually 2560x1440 one to the highest DPI, which I guess is the native 5120x2880 and I just pull it closer and/or lean closer to it.
I think for programming the 2560/1600 = 1920/1200 = 1.6 or the 3000/2000 = 1.5 ratio is a reasonable middle ground. Unfortunately it's not a common ratio among big but cheap monitors...
The extra head movement required to look into the corners of a 27" or even a 25" vertical monitor definitely puts a little strain on my neck compared to a 25" horizontal one, but with frequent posture changes and breaks, while a bit tiring, I don't feel it caused any permanent damage.
At least I hope it's not the reason why the middle of my upper back feels like falling apart any minute... Maybe that thai massage, when a lady was walking on my back for half an hour was a bit too much? :D
Yes! I have the same and despite spending thousands of dollars trying to find a better monitor it's still best solution I've found. I can keep 4x80 column files open in vscode or 8 if I split them vertically. And with a single monitor I can still easily use a KVM switch.
I like to sum up this debate like this:
With multiple monitors, you're confined into smaller, "hard-coded" boxes that can't be changed. With a single large screen you have much more flexibility.