I get that, but look at the 5k market, 3 or 4 options after how many years?
And, the Dell 6k display sells for what, 2200 US$ (it's not on par with the XDR) or so? add a Mx processor and other needed parts to make that an iMac and you're well above $3k, more like $3.5k entry price - what is the market for that?
Exactly, the point is it is NOT on par with the Pro XDR, so it can be sold for way, way cheaper. It's a 6K display that retails for US$2399 but which can be purchased on sale for under $2200. The Pro XDR's 1600 nit display consumes a lot of energy and heats up significantly, requiring a heavily ventilated design to compensate, and most users don't need or even want that. A 600 nit non-HDR display makes a lot more sense, esp. if Apple were forego the 218 ppi spec and release something like 5.5K at closer to 200 ppi (which is still "Retina").
As for an iMac, it'd have to be more expensive, yes, but certainly not anywhere near Pro XDR pricing. The Pro XDR is a low volume high end niche product, like the Mac Pro.