15 years is a good long stretch!

I've been buying them about that long too. And a lot has changed since we started doing that.
If there's a determined community buying full adventures, whatever the purchasing venue (I personally buy from all venues, including Big Fish and other online portals and online stores, the individual developers, and the brick and mortar stores), I'm convinced that developers will keep making them. One trick up our sleeve is that I think the developers enjoy creating them almost as much as we enjoy playing them.