What makes one a pro computer in 2023 could come down to Apple simply saying so... perhaps supported in a more tangible way by putting a Silicon PRO chip inside.
What makes retina screens something? Vs. retina HD? Vs. Liquid Retina?
What makes some products "Air" and some not? What makes some product + (plus) and some not? What is the real plus part?
Some-to-most of it is simply branding it as such. Apple says so, so it is so. Whether there is any/some meat on those bones or not may not matter. Maybe Apple rolls out something tagged "extreme" or "enterprise" or "kukukachoo" and it will be promoted as such and probably accepted as such solely because that is what Apple chose to call it.
What advantage would an iMac Pro have over Studio + Monitor? Some people- seemingly a LOT of people around here- care about
aesthetics. iMac "all in one" doesn't necessarily need a bunch of other things/wires connected to it. "Look Ma, no wires!" seems to be a thing fairly important to some people. Some will make a case about just plugging in one power cord and being ready to use it vs. having to hook 2+ things together and plug in 2+ power cords. Etc.
OK, what about vs. the also, all-in-one 16" MBP? Bigger screen: iMac offers a much bigger screen than 16" MBpro. That one is very obviously tangible.
If you want to jack the conundrum: why 14" vs. 16" MBpro? Why "fill in" that gap(?) at 15" (Air)? Why is there a 13" pro and 14" pro? Why 13" pro and 13" air? Why Mac Mini PRO and Mac Studio MAX? For the vast majority of uses, those are- in effect- the same. Why 5 families of iPads that all run the exact same OS and apps? How does that make any sense? Why create a (NOT technically) "new" yellow iPhone a few months ago that is EXACTLY the same as the existing ones except for a color? Answer to all such questions: apparently it makes Apple more money than NOT having those similar/same offerings. And that's enough for Apple.
Your last sentence is an eye of the beholder one. Where was that argument used against Apple when Apple was pushing iMacs? Why do we only seem to see faults in Apple offerings AFTER Apple chooses to retire them (or in the rumored Goggles scenario, BEFORE Apple themselves confirms existence, shows them, blesses them for the flock, and then it is good to all

)?
I recall very passionate arguments/defense FOR butterfly keyboards even as problems were piling up. I recall very passionate "notch is iconic" and similar BEFORE dynamic island. I recall extraordinarily passionate arguments against phablet phone sizes FOR YEARS while Apple clung to 3.5" and then 4" as "perfection."
Right now, there is a crowd still passionately arguing for Lightning vs. USBc because "Lightning is superior, etc" but those people don't seem to pop up in the threads for all of the products where Apple has already embraced USBc, ripping Apple for doing so. Apparently where Apple products already use USBc, it is fine and right and ideal. And where it is not used yet, it is stupid, fragile, prone to expensive repairs, etc. IMO: once Lightning is gone, I suspect "good riddance" will quickly arrive... as it quickly flipped for the 30-pin crowd... and firewire before them.
I very happily went separates myself after a very long time with iMac 27. Why? Because separates offer the "don't throw baby out with the bathwater" benefit at the end... when- IMO- it makes ZERO sense to have to toss a perfectly good monitor because the tech guts conk or a corporation makes the computing parts obsolete by "vintaging" choices. But there are plenty of people who seem to think that AIO iMac "bigger" is ideal for them. If so, it doesn't need to "make sense" to those who feel differently. If there's enough buyers of anything that doesn't make much sense to some people, that's still an opportunity for Apple to sell more Macs.