I don't think that the people who only go for free dining will pay for the plan. People will just use the counter service restaurants and maybe pay for the odd table service restaurant if there is something they really want to do.
Hopefully if it does put people off, Disney will have to think long and hard about inproving their service to what it once was to try to encourage people back again.
At the end of the day Disney had two aims, to make their resorts busier in off peak seasons and to tempt people to eat on site. Well the parks are busier, but whether people will actually pay for the Dining Plan is yet to be seen.