As a local, I would not have any issues with on-site guests only being allowed to make PS'. SoG and S/D guests would be left out of the loop. But ADRs for on-site guests only could always become a perk.
As a local, I can eat there most anytime, anywhere, or even off site or at home. No biggie to me.
BUT, IF they would ever even consider this route, I think it should be on-site guests and those with DDE memberships (ao AP holders as someone else mentioned). That could just be thrown in as another free perk for the DDE/AP.
Disney WANTS and almost always DOES everything they can to make a visit magical. They don't want unhappy guests who will not return. They want HAPPY guests who become addicted and never want to leave, and tell all their friends and family what a great time they had!
It is all about customer service.
But if the duplicate ADR situation gets carried away, and they get too many complaints, it will change.
And it is not unusual any longer for walk up guests to be turned away. Le Cellier is a great example.
I would like to see only about 75% max as booked ADR's. It would be nice to have SOME walk ups accepted.
I think those on the dining plan should have top priority because of the limited nights credit situation.
Of course a NICE change that would help would be more frequent and better counter service in all of WDW.

If the counter service was more plentiful and GOOD food, then too many sit down would not be an issue.
Heck, Disney won't even keep the taco CS by pirates open.
Put in a Cheesecake Factory in a little soda shop setting (like at USO) or other just as good eats, and table service would open up.