Well, in my particular case, I use an M1 Max MBP with a 2K monitor that I already had when I bought the MacBook.
The problem with 27" 4K monitors is that you can't have integer scaling on them. If you set the scaling factor to 1x, everything will be too small, if you set it to 2x, everything will be huge, and macOS can't properly do fractional scaling because what it actually does is render everything into a 2x framebuffer and downscale that for output.
And besides, only supporting HiDPI displays doesn't mean one can stop striving for pixel perfection. I hate SF Symbols icons because they're sizeless. They're an abhorrent blurry mess on my monitor but they're also not all that sharp on the MacBook screen. If you notice it once, it'll haunt you forever. Sorry. They do look fine-ish on iPhones though because those use OLED displays that lack the notion of a pixel grid anyway.
