I say this as a guy whose dream gun is a Thompson. I don't see the point of pistol caliber carbines anymore. Other than range toys or for optimized competitive games.
I remember the 90s when an MP5 was the end all be all carbine until we remembered that an AR or AK could do everything it does better. And close quarters you probably still want a shotgun.
9mm? They could have made a legit rifle out of 10mm, .357, or .44. Enough to be a takedown hunting rifle with range.
I just don't see it. Maybe I'm missing something.
I don't think you are missing it, you have all the pieces above, perhaps just not weighted in the places I would.
Lets start with the issues of pistol caliber long arms. You are right, rifle cartridges can do everything a PCC can do and better. Especially hunt and poke holes in people. However, I think the stroke of brilliance was to make it in 9mm, capable of taking one of the most ubiquitous pistol magazines and making it relatively low cost. The old Ruger Police Carbine was a Bill Rugerism that still couldn't see police using AR family rifles. This is a plinkster, which is why they didnt start in one of the .40 calibers or bigger. It is a little slow out of the starting gate since .22lr is back on the shelves, but bottom line is that 9mm is cheaper than all other (more powerful) pistol cartridges and nearly all centerfire rifle cartridges. Plinking for most US shooters is inside easy visual feedback range, you generally dont need a centerfire rifle for that. You don't need rifle steel. More and more outdoor ranges and even individuals are buying steel and rifle steel is much more spendy than pistol (by the way some pistol steel cant take high speed .44 Mag).
There are probably nearly 100K members of IDPA and USPSA, so competition is also a valid market. The draw of the pistol caliber carbine is that you can shoot the exact same stages that you do for pistols, ARs would require you to at least change how the steel is utilized. As I said before, the reliable pistol caliber carbines offered right now tend to be expensive, so this as a modest priced one will be tempting.
Defense and hunting is not a huge market. Those who want to use pistol calibers to pot animals already have lever action pistol caliber carbines and obviously pistols. Those who want to defend themselves with pistol caliber long arms already have the Beretta Storm. So it is probably smarter for Ruger to build the platform's market elsewhere first. They choose 9mm in that it is the cheapest round that cannot be dismissed for defensive application. They sell it as a plinkster, but keep it in a round that keeps their foot in the door for those that have pretensions of defense. This is something a .22lr has a hard time doing.
I am thinking about the ammunition expenditures I have had in the last 15 years as a shooter.
LE (2 years): 2 (dispatching animals hit by cars)
Hunting: 12 (not a regular hunter)
Plinking: Uncountable, in at least the 6 figure realm all calibers included
So an affordable plinking centerfire pistol caliber carbine may have a piece of market share. Anecdotally, pistol-forum.com, which tends toward a haven for serious students of the pistol and heavily tend toward Glocks normally scoff at PCCs for many of the reasons you mention. There was a lot of thoughful "hmmm" reactions when they heard that it takes Glock mags.
None of this is to say that the economic viability will be there for the Ruger PCC. People have always liked the concept of the pistol caliber carbine more than it seemed to be sold and adopted. But considering the market right now, this is intriguing and seems to have a better shot than most recent attempts. Something more than a Kel_tec sub-2000 or Hi-Point, less than a high end pistol caliber AR.
Bottom Line: Not for poking holes in living things, it is for cheap plinking that also cannot be discounted as a defensive gun.