Jury's still out
Interesting question! From a "big picture" perspective, generally, it seems to me, two things are needed to make something collectible:
1. It's gotta be a good example of whatever it is. It's got to have inherent value and worth. And ...
2. It's got to be scarce. The value'll never get up high if there's a zillion of 'em.
But there's another situation that the above don't quite cover. What if -- even if the item wasn't the greatest ever -- nothing comes along afterward that's as good or better?? The TR6, for example, wasn't all that great a sports car. But who knew that we'd never see anything even remotely close to it until the Miata?? Nobody! And by then, everything (safety standards, regulations, fuel requirements) had changed. So the TR6 is enormously collectible, mostly because it wound-up being the last of its kind ... essentially by chance.
I cite this because we've potentially got all three situations with the ST3. First, it's a superb sport-tourer without a doubt. (So we've got that goin' for us ... which is nice.) Second, production numbers were relatively low. Plus, although nobody could have predicted it, we're looking at the same kind of intangible described in the paragraph immediately above: nobody's made anything better yet.
The fly in the ointment, as I see it, is the sport-touring class itself. It's a compromise category. And people don't tend to get wistful and misty-eyed over compromise categories of anything.
I bought mine used and, based on the eBay prices I see, have never lost a dime of value. So I'll keep it and keep riding it as long as it'll have me. What'll it be worth to somebody else someday?? I'm sure I can't say.
But I'll keep it parked in the garage, sharing a bay with my '97 air-cooled 993 cab 6sp (w/ 15,000 mi), just the same!
Best wishes to all of us!