As I said elsewhere, that Pedersen design looks simple, but it isn't
It's actually pretty simple if you get a chance to examine one (Model 51 or R51). It's also pretty simple to see how/where Rem screwed up to cause the issues most commonly encountered;
-Cheap mags
-Rough-finished machined parts
-Rough-finished MIM parts (at least these seem to be dimensionally consistent, though)
-Rough and/or short-chambered barrels
I defy a 1911, Glock, M9, or any other pistol design besides an open-bolt blowback to be constructed so poorly & still function. Tokarev's I've seen are much nicer on the inside than the R51 (and yet mine still ran for the most part, and even fairly reliably after some tweaking I shouldn't have been expected to do)
"It was to be the same size as the original, which would be a pocket pistol about the same size as a Colt M1903 hammerless."
Where'd you hear this? It's pretty obvious from the offset barrel/slide muzzle and long/wide magazine that this gun was intended to be a neo-M53 in 45acp at some point. Maybe for a 45acp one might be able to consider it a 'sub compact' but yeah, mostly marketing there (though it does conceal/carry like a much smaller gun, due to there being lots of round corners and a thin overall profile)
"If they made the PX4 with a single colum version it would make a great one"
There's no point in doing so; the rotating barrel necessarily makes the action wide enough to justify a double-stack magazine (it's not like a 1911 where the magwell is fatter than the slide)
"as with the Luger toggle lock, the Steyr GB gas-delay system, and the Benelli B-series intertia lock, there's a reason that other gunmakers haven't jumped all over the concept."
Pretty much all those except the Steyr were what you'd call premium pistols back in the day. Arguably, the M51 was, as well, considering its competition were all simple blowbacks. The Steyr was doomed by very R51-like failures in the suspicious knock-off Rogak feeler-distribution prior to the GB's release (there was also this certain new gun called a Glock 17 that kind of ruined all the best laid plans of mice & Steyr to market a large, ungainly steel pistol)
There's also the FN Five-seveN pistol, which uses a sort of hybrid blowback/delayed blowback. Basically, the barrel is allowed to be dragged back a short ways by case friction early on, loosely coupling the two together like a sort of Thompson Blish Lock long enough for the very brief pressure impulse to drop to safe levels. The addition of some barrel inertia also helps keep the slide velocity low. Supposedly messing around with or removing the teflon coating on the brass impedes this function and leads to unreliable or unsafe operation (in reality it's more to prevent galling in the PS90 magazine). Once again, a fairly premium pistol, though I doubt it really must be so.
Also the HK P9s, which was a miniature MP5, fluted chamber and everything. Again, fairly premium firearm. No reason I can see that all of these MUST be premium guns vs. made more modestly, but it was how they were introduced to the public/military, and a big part of the reason they failed to get the market exposure of the 1911 or later Hi Power.
Not familiar with a pistol using the inertia lock of Benelli/Franchi/etc shotguns, but it'd likely require a lot more felt recoil than short recoil actions (the gun has to move a certain amount in space for the system to move, so the opposite of guns that fail due to 'limp wristing')
TCB
