The problem is, because the GF (and other non-DVC Disney resorts) is an entity outside of DVC, the compensation that GF gets from DVC when members stay there on points is re-negotiated every year. And the GF will likely want more money for those stays ever year. And so, the number of points per night required to stay at the GF goes up pretty much every year.
So, right now, if you bought a 200 point DVC contract (roughly what you'd get for 20K), you could stay at GF for 4 nights a year, IF you stay in the cheapest room, during the cheapest season on a weeknight. The point costs will likely go up next year, so let's say they go from 48 to 52. Now you can only stay for 3 nights a year. See where this is going?
And, as I said, that's only for a garden view room during the cheapest seasons. You want to go from May-July? That's 54 points a night. During a holiday season? 76 points a night on weeknights. 87 on weekends. So, if your routinely stay at the GF during the second half of December, your $20,000 would buy you 2 nights at GF a year.
For comparison's sake, with my 150 point contract, I can average about 10 nights a year in a studio at any DVC resort (figuring 12-15 points a night). And that's not going to change much, if at all, for the life of the contract.
That's where the value lies in DVC.....using it at the DVC resorts.