21 years is closer than you think:
First off, in terms or resale value -- the expiring resorts won't suddenly lose their value in 2040. Their re-sale value will start to decline significantly long before that. I suspect it will really be felt in the re-sale value within the next 10 years.
Second, as you said --- look at all they have built in the last 21 years.
Disneyland Tower is coming... and not unrealistic to think Reflections will eventually come back (though likely delayed 3+ years), and potentially other resorts. So right now, yes, it's 1 restricted resort and 14 "unrestricted." But this 1:14 ratio isn't going to stay the same for the next 21 years. 2042 will see a big shift, but it will start to shift before that. By 2030, only 9 years away, it wouldn't be the least bit surprising if it's more like 3:14 or 4:14... with the restrictions on the newest 3-4 resorts.
As Seth said, it's a different equation for different people. If you're planning on holding a monorail DVC for only 10-15 years, then being limited to the original 14 resorts may not register any significant pain. If your plan is more like 20 to 30 to 40 years... A re-sale purchaser may look back at regret in 25 years, wishing they had bought direct when it was "only $200 per point."