I'm sure at some date in the not too distant future, there will be those who will specialize in collecting plastic guns, for example, Glocks, and will want to have a complete set of everything Glock makes or made. From a historical point of view, I see nothing wrong with that; In fact, I think it would be very educational to see something like it.
At some time, as technology advances, they may even become collectible as being representative of a bygone era when guns were still made of plastic and not "pressed fiberboard" (for example).
From a practical viewpoint, however, there are no really significant events or epochs associated with plastic guns as opposed to, say, Winchesters, which, right or wrong, are invariably associated with "The Old West," or 1911s which are associated with any number of wars that this country has fought.
For a gun to become "collectible," it has to have some history or "romance" involved with it. Plastic guns were developed as a manufacturing/cost expedient, and, as such, most likely will never have any significant collectibility any more than a garden variety Model 10 Smith & Wesson will.