A fair point, as there are third-party 24-inch panels with 3840x2160 resolution. Then again, because there are third-party options, that might preclude Apple from actively shipping one since it would only be usable as a standalone display (whereas they can use the same panel in the iMac and a 5K TBD).
Though honestly I would rather go with the LG UltraWide 34" at 3440 x 1440 than a 24" "retina" display.
Yeah I just saw that utra wide lg display and was gonna post that I hope apple makes a screen like that