I see where you are coming from, I guess it comes down to costs in that potential future. If Disney removes all benefit except trading between resorts, I still feel like that isn't a great incentive as you can already stay at a ton of resorts paying with cash, more resorts than you can if you are part of DVC even! A big draw of DVC is the potential to stay at nicer accommodations for a fairly static cost over a long period of time, plus you get access to reduced AP pricing! In your comment earlier, the future is a world where AP pricing can be dropped, so all you have left to entice people is access to resorts they can already stay at for (definitely high) rack rates, or just keep going to CB or AoA or whatever other far less expensive resort.
I just can't get past that the future you outlined doesn't seem sustainable, there is just too much of a "death by a thousand cuts" feel to it. People around here are already wary of RIV due to resale restrictions, you paint a picture of people willingly buying direct when those resale restrictions are at every resort, no more AP discount, no more MM, and ever increasing dues. Sure they will still sell DVC to some extent, but no way it retains its popularity, especially when the biggest supporters like this and other boards are filled with members or former members telling everyone to stay far away.
The only way I see that world working is if they add other benefits, like an unlimited or at least very generous park hopper, or some other big grab that really wouldn't cost Disney much but be super valuable to DVC Direct members.
But again, I know you have way more experience with all this than I do. My view just comes from my personal feelings of it. Already have trouble seeing a strong value proposition in DVC although I know it's there, if they completely dump the other benefits I can't see how they get new members at the current rate. Guess we will see!
I've heard a million times, "people will never accept that change!" -- Not just DVC. Within a Disney context, heard it about every change to fastpass, every change to the finding plan, heard it about every significant price increase. In the end, after some initial friction, most of those changes become accepted.
Let's assume that the resort-trading-ability becomes the only direct immediate benefit of booking direct. You still have the cost-savings inherent in DVC.
So why buy DVC instead of just paying cash rooms at the hotel of your choice? Even without an AP discount, the answer would be the same in 2030 or 2040 as the answer is in 2020 -- To save money in the long term.
In this future -- You can buy direct, which allows you to save money while booking 15+ resorts. Or you can buy resale, save even more money, but at single resort.
Getting rid of AP discounts or moonlight magic isn't going to change that equation all that much in the long term.