Yes, I think that's not a major concern. Indeed, the concern has been that the restaurants are just too crowded as things are now.
That's one reason why I tend to think that Disney feels it can afford to trim back the dining plan. If they are consistently turning away guests at restaurants, they can afford to lose some on the dining plan and still fill most of the tables each night.
The only major impact I see is that Disney will need to find some other type of incentive to fill hotel rooms -- not restaurants. And they have lots of choices. The effect of the (for pay) Dining Plan on hotel room bookings probably isn't as large as the effect of the free Dining Plan anyway, and the free Dining Plan, even with the changes, probably will have pretty-close to the same effect.
From what I've heard about occupancy, they don't necessarily need the DDP to drive up occupancy. The "free dining" may still happen in the Fall during those truly dead periods, but other times I suspect they're betting on maintaining high occupancy without sacrificing profits via the DDP.
Regardless, Disney has nearly a year to gauge the impact of these changes before they make any announcements for '09. If the impact is not as favorable as is currently projected, they can tweak again the following year.