I said it a while ago that the grat would be gone, but a lot of people wanted to aruge otherwise.
Well unless disney is dumber than I thought, they should simply REQUIRE a credit card be linked to guests IDs, thus when they are swiped for the dining plan, it automatically records the quests credit card number, and is ready for a tip.
Service won't drop overall, servers may just start looking upon dining plan guests as second rate guests if the tips drop. Ex: A server at Ohana's gets two tables at the same time, while they are extremely busy.
One table comes there yearly and loves it. That should be a safe bet for a decent tip if service is on par. The other table has never been there before and is on the dining plan. The server has already in past weeks determined that tips are declining per dining plan guests. As stated, the server is very busy, and doesn't have time to get perfect service to both guests. Its common sense to go for the table not on the dining plan.