I think most veteran visitors to WDW have seen at least some decline in quality, not only in the dining options, but in general.
When we first started going 24 years ago we used the "World Adventure Vacation Plan," a.k.a. "Gold Key" plan which included three meals a day at TS restaurants (dinner shows included). It was not, in any way, "
free dining." It was more of a convenience that you paid for and if you didn't know how to use it you actually paid more than you normally would for dining.
I'm sure any current "free" dining plan or any dining plan is not really free. Someone is paying for it either directly or indirectly and in many cases the quality of the food has gone down enough for so many people to notice. We've noticed it, but there are still a lot of good meals to be had in WDW. Realistically, a $12 burger had better be a really good burger or people will find alternate places to eat. Sure, Disney may make some profits for awhile, but they will lose in the long run. I have seen a trend in restaurants no longer serving breakfast, and in some cases, lunch so they don't have to staff for meals that weren't filling the seats.