They should make a 5.4 inch version with "Pro" features. Some of us like smaller phones like the SE for their size, and not necessarily for their lower cost.
iPhone 9 4.7 IPS screen , A13 , iPhone XR camera , touch ID , no 5G ( $299-399 price zone)
iPhone 12 5.4 OLED screen , A14 , iPhone "12" twin camerass , FaceID , 5G (sub 6Hz probably skip mmWave). ( $599-699 price zone). and maybe even wireless charging.
Uptick on screen , processor , camera (in number and quantity) , biometric sensor , and cellular connectivity. What didn't they hit? (even more so if wireless charging is present ).
The "pro" class would be a few more tech differences but there would be a very substantive gap between the iPhone 9 and the iPhone 12 even if they have approximately the same LxWxD dimensions. (wireless charging may mean there is a depth difference. or that 5.2 gets by with a smaller battery than the older processor and IPS screen variant because components consume less power. )
One would be the affordable "Small iPhone" and the other would be the upscale "Small iPhone".
They'd only miss out the the maximum tech in the smallest possible package. That probably is a much smaller market than these two "small iPhones" put together. a sizable number of Folks who put a very high premium on the three (or more) cameras would make the trade off for the 6.1". Broad, deep market for a $999+ phone in the "small phone" space.... probably not (at least relative to the combo of the other two).
P.S. that iphone 12 spec list above is better in most respects than the iPhone 11 Pro. It is smaller ( and if that is a priority better). Better processor , Possibly better FaceID , better Cellular radio . There would be a narrow subset of camera performance gap but it would be a high end camera system relative to the overall cell phone market. Apple may not slap a "Pro" adjective on it, but the price is quite high ( if "Pro" means high price) and performance is there. In terms of computational performance it would faster and $300 cheaper than the iPhone 11 Pro. Lots of people are going to walk away from that?