My Disney math is that I have three girls in 8th to 11th grade. Over the next 8 years, we will be buying cars, college tuition, insurance, etc. The yearly cash WDW trips are going to get tougher to justify, especially with the way costs go up every year.
We will be in a good spot to buy direct this winter to get our foot in the door, so it makes perfect sense to lock in much more affordable future annual costs.
What you're describing is exactly why we bought DVC when our kids were born. We wanted to know that when we hit that really expensive time we'd still be able to take vacations even as the kids get more expensive. We figured the kids will only get more expensive as they get older!
