It seems HIGHLY unlikely that there will be a point when there are only pay to play courses. Sure, there may be more than there are now, and that will probably be a good thing, but they won't be removed from public parks and the hiring of staff to collect fees on courses in public parks will still be extremely cost-prohibitive.
And the only thing that will drive disc prices higher like that, other than inflation, would likely be either a higher quality material that is also more expensive to produce, or a lack of the current material, which is much more likely. There is simply too much pushback from the current disc golf community to allow for a significant, unwarranted, price-hike.