Gosh, I hope not. I like a buffet once in a while, but I would much prefer to pay a plate fee than see more buffet pricing.
By "buffet pricing" I mean charging per person. The restaurant could be buffet, all you care to eat or just a set menu. Disney restaurant pricing isn't really based on the food that's consumed. We're paying for the atmosphere, characters and location.
I don't think Disney will want to enforce a plate fee.
edited to add
I don't think anyone in this thread speculated on an answer to the OPs question, how long before they stop letting guests share?
This is only speculation, not even a rumor, but I'll predict that sometime before the next free dining program at least one restaurant will experiment with a policy that requires all guests pay. I don't know how many guests really book CG or SCI-FI for just dessert but those are both possibilities.