There are several reasons behind my premise that people's spending decreases as the number of DVC stays increases.
1. Food. I guess it depends on what you mean by 'cook'. We don't 'cook' on vacation, as in lots of pots, pans and potato peeling. However, we do pick up some things at Winn-Dixie for breakfast and lunch. It's only DW and I, but try this: we have coffee and pastries in our room for breakfast every morning. That's what, $10 / day Disney doesn't get (and about $7 day in savings). We also pick up lunch meats, mustard, soft drinks, bread & chips for lunches. That's about $20 / day a Disney CS restaurant doesn't gt (and about $15 / day in savings). We rarely eat more than one or two dinners on-site; with a car, Outback, Pizza Hut, Applebee's, etc. is a better deal - and often, better food. That's upwards of $75 / day Disney doesn't get. And we did NONE of this when staying in a Disney resort room. So far, that's over $100 / day we no longer give Walt.
2. Souvenirs. We always pick up one 'cheesy' souvenir everytime we travel, and we're already running out of room on our shelf

. I guess that after the 10th, 15th or 20th trip, most people just don't need any more Poohs, Eeyores or mouse-ear antenna caps. Now, maybe as rabid Disney fans, some of you do - but that leads me into a key point:
DVC DIS OWNERS ARE
NOT REPRESENTATIVE OF DVCers IN GENERAL NOR RESORT GUESTS IN GENERAL!!! Sorry to shout, but so many people forget this. By definition, we are in the top 5% (1% ???) of fanatical Disney guests. We actually take time out of our day to post & read the DIS - because we enjoy it! Don't get me wrong - I think that DIS folks are some of the finest people I've met. Yea, us!!! But we are not the same as the vast majority of DVC owners who don't even understand 11/7, add-ons, banking & borrowing - some folks even let DVC point expire, unused, unbanked and unrented

. Think about it; when some is talking Disney in the office or at practice or at the store - who is it that everyone else turns to for the answer because "S/he is a real Disney nut" (and has more Disney stuff on their desk than anyone else)?
So, I'll grant you that some DIS DVCers probably spend more than resort guests trip after trip after trip. But, on the whole, with our kitchens, our exisiting souvenirs and a gradual " been there, bought / eaten / rode that" attitude, I don't see DVCers as cash cows once they've paid off their points.
IMHO - YMMV
PS to NMW: I bet that you'll find yourself able to have just as wonderful a trip as you always do with at a slightly lower cost. Hey, then you can buy more points!