I guess the economics of selling a relatively low-end machine with a high-end display for a bargain price have changed - 1440p started out at a huge premium but rapidly dropped as adoption increased. Apple jumped on the 5k bandwagon early, but the format completely flopped in the PC market and never came down in price. I suspect that the low-end 5k iMacs must have been one of their lowest-margin products. Plus, desktop sales are probably continually declining overall, so the economies of scale might not be there. Anyway, we have the 24" iMac which is the second largest-screened iMac ever - and it has a very good display.
It depends what features you feel are most important: If you must have a ~220ppi display then there's nothing else on the market (Dell and Samsung have announced 5k displays but last I looked we don't have prices or availability and they're probably - at best - comparable, if not identical, panels to the Studio Display). Personally, I don't mind dropping to ~160ppi if it means (eg) I can get a dual, matched display setup for half the price of a single SD, but there's no denying that 220ppi is the "sweet spot" for MacOS and visibly superior if you want to pay the price.
Refresh rate - yes, you can get displays with faster refresh rates if you want to sacrifice resolution, but the #1 market for those seems to be FPS gaming. Apple has a leading gaming platform: it's called the iPhone - the Mac has never pretended or really aspired to be a gaming machine.
Size - again, bigger displays are available but not at 220ppi resolution. Dell have a 32" 6k display in the pipeline which will doubtless be a bit cheaper than Apple's Pro XDR but there's no evidence that it's going to be better (as far as I can tell it doesn't have local dimming). Plus... you could maybe make a 30" iMac with narrow bezels that wasn't noticeably larger than the old 27", but the 27" is already a massive slab to have on your desk that make sit really awkward to get at the ports, so going larger doesn't make ergonomic sense to me - many people are going to want it wall mounted or on an articlated arm, at which point you've got all of your USB etc. devices dangling off a "flying" display...
Do you need HDR? Actually, a lot of people really don't, and some that do will need better HDR than you get with a few hundred local dimming zones. (Apple had a bit of brass neck when they compared the Pro XDR to a $20,000 dual-layer HDR reference display). OLED would be great, but until its 100% clear that the burn-in issue is solved you probably don't want your $2000+ computer welded to one of them.
As I see it, the diversity of displays currently on the market just further justifies the Mac Mini/Studio + whatever display you choose model. Choosing an Apple display gets you very close to the iMac experience if that's what you choose - and if you wanted a higher-end iMac/iMac Pro level of power it doesn't really cost any more.
Some people seem to want/expect a 30" XDR ProMotion iMac starting at about $2000 to replace the old entry-level 5k. I wouldn't hold your breath. Maybe Apple will make a new iMac Pro if a suitable display technology comes along but it's not gonna be cheap.
