I think most people would answer that you should of course complete your collection. And if money is no object, then that would (naturally) entail buying three pristine examples of each caliber, in each size, for each year of production and each finish variant produced that year - not to forget the "luxury" variants like the elite, expert, tactical, etc., which you would also buy three each of, for each caliber and year available...
But seriously, it all comes down to your preference. Obviously most people are going to have a reason to set the goalposts for their collections a little lower than that. The USP40 Compact is one of my favorite .40S&W pistols I have ever shot, and I'll probably never sell it because, after living through the Great 9mm Shortage of 2013, I always want to own at least one .40S&W handgun from here on out. The USP40C is great because it's relatively concealable and has very good capacity, while handling the recoil of the caliber way better than average. That said, if you already own the 9 and the 45 of the same gun, I'm not sure I can make the case why you'd need the 40 too without getting into banned caliber discussion. Ammo availability is a very strong reason to own a .40, but I'm not sure if you own other handguns in this caliber as well.
