But those are just the reasons it could be (or could become) a collectors' item. If they had made a million of them, there would be no collector interest.
I should make my position a little more clear and a little less murky!
I'm not saying they don't have part of the recipe for collectibility -- certainly they do. Noticeable role on a popular TV show, involvement and push from a wildly popular gun scribe in the industry, cool factor, and other bits and pieces that help build the resume for a collectible piece -- one of which is obvious exclusivity and the short supply of them.
However, my point is that there are absolutely NEGATIVE characteristics as well, and very well documented, and those are no small thing. The fact that some slides are KNOWN to fracture and erupt... making shooting any of them questionable? As well as the hi-jinx with the original shipping of magazines that may not have been a straight-up planned screw-job, but was perceived as such? Not to mention the fact that others furthered the idea and platform and went ahead and made actual working guns that made the cartridge viable even when the debut pistol certainly wasn't? The CZ-75 heritage was, at the time, entirely out of reach in the western market. Notice the price on Springfield P9's when you see them now.
My take is that the boatload of negatives associated with that particular blip in history should (in my opinion) temper the heights of collectibility that it could or should ever reach. But it is an organic thing we are talking about and often will not be explained.
Also, you used the term "a million of them" and while I understand it is merely an expression and not to be taken literally...
...they made over a HALF million Python revolvers and if you were writing the newest edition of the print dictionary that nobody uses anymore, you would have a picture of a Python next to the entry that said "gun collectible" and I don't think anyone could argue with you in any way for doing so.
There are not really any hard and fast rules for what builds and carries the market for things deemed collectible, only guidelines we have witnessed and noticed. My take is that the kind of prices the OP shows for a Bren Ten do, IMO, shatter many/most of the precedent we've seen. Most of the short-printed guns that bring HUGE collector value share one thing almost all the time: they are extremely good guns.
I submit that the Bren Ten was
not.