It's hard to "convince" someone unless you know what their reservations are.
If they flat out just aren't interested, then there's not much you can do. If they like the idea, but not the large upfront costs, you might could introduce them to the resale market where you can get fewer points. You could also mention the ability to rent/trade/borrow/bank to either offset some of the yearly costs or to plan longer or bigger (larger rooms, more people, more rooms, etc) trips. Finally, numbers can do wonders. If they actually see the numbers of how
DVC could breakdown over the years, it might catch their attention.
Sometimes people just don't realize how they could benefit from it. I mean if they are going twice a year... I'd find it hard to believe that they couldn't save unless they were staying at the cheapest hotels in the WDW area.
For their next trip, why don't you offer to rent them some of your points so they can actually stay at a DVC. Better yet... all of you could go.

Take a day to go around to some of the other DVC's and show them around. If that doesn't do it... nothing will.