I agree with you both, but Apple has these stupid self made standards like “retina” which means they won’t drop the ppi to go from 4K 21.5” to 4K 24”. Even though ironically both have very high image quality. The only chance of a 24” size would be if they created a smaller 21:9 iMac to complement the rumoured 31.6” 6k2k.
I don't agree that keeping everything "retina" is stupid, but there's no reason Apple couldn't make a 24" display with some custom retina resolution. The 21.5" iMac already uses a rather unusual resolution in 4096×2304 and so does the 12" MacBook in 2304x1440. 24" 16:9 at 218 ppi is about 4552x2560, seems like a perfectly fine resolution imo.