Ah the impossible question LOL. There are so many variables this is almost impossible to answer. Will Disney over supply? probably. What inflation rates should we use? who knows. Should you use other timeshares historical action as a guide for DVC? Maybe, but I think there are other actions for DVC that are more important than that last issue.
Where I feel DVC has an advantage over a lot of other timeshare systems is that it has an easy and obvious point of cost comparisom ie Disney Hotels. I calculate what I think DVC points will be worth using a likely inflation rate for Disney's other hotels. My logic behind this is, that even with just 3 or 4 years left of DVC, if you are planing to visit WDW twice in that 4 year period and your accommodation bill is going to be for example $10,000 per trip ie $20,000 total. If you can buy DVC points ( and by banking and borrowing cover all your needs) that will give you accommodation of a similar or better level than you would have paid cash for, there is going to be a value still to DVC purchases. I would think anything less than 50% of hotel room cost is going to get snapped up, somewhere in the region of 70-75% is probably a little on the high side. Ultimately, the value of DVC is going to reflect what the cost of staying on WDW paying cash is going to cost.
If you take a hotel room that at the moment costs $150 a night ( IMHO that's a reasonable guideline for a hotelroom of DVC quality including any discounts etc) and apply an inflation rate of 5% per year you come up with a cost of just over $1000 per night in 2042. Your dues using a starting cost of $4 per point and going up by the same inflation rate comes out at $27 per point. If we use 130 points as the cost for that room, which is arguably a little high your cost of dues each year is $3500 for the week, about half the cost of paying cash. On this calculation the saving of DVC over cash would be about $14000 ( $3500 per week over 4 years) or $107 per point. Obviously no one is going to pay $107 in order to save $107 but I think even at that late stage in DVC's lifespan anything less than $60 per point looks cheap.
Using an inflation rate of 8%, which was the historical figure for Disney hotels PRIOR to Sept 11 , the effects of which ( deep discounts on rooms) I think needs to be ignored for the purpose of this calculation gives you a room cost of just over $3000 a night and a dues cost of $80 per point. The dues cost of your room is about $10,000 ( again about half of your room cost) . Using this inflation rate the saving of DVC is about $40,000 ( $10,000 per week over 4 years) or about $307 per point. At that level any cost of DVC at less than $150 per point looks to offer a saving that would be worthwhile.
As Dean correctly points out, inflation is the biggest imponderable, but DVC's legal set up, in that Disney can't make a profit out of the dues, they are only to cover the cost of running the operation make me think that rises that are hugely above the rate of inflation are unlikely. The other issue that has been touched on is if/when Disney stop selling DVC, the level of marketing will obviously drop as will the level of general knowledge about the product. This lack of knowledge among the general public will undoubtably slow down the numbers of people interested in buying into DVC (through resales) but obviously there will be no supply from Disney. Again it's a question of supply and demand, both will drop in this scenario, depending on which drops the most the price will rise or fall accordingly.