I have to say, I agree with the noticeable decline in Disney Dining. Portions have shrunk, quality has decreased and overall there isn't the fun variety there once was. There are still exceptions like Boma or California Grill where I still feel the magic, but we are tempted to try more off property places, obviously not every time, but staying on property for every meal doesn't have the allure it once had for me. This trip, for the first time, I didn't want to have a Disney breakfast each day, many days I was content with a granola bar or cereal in my room just because the food isn't as amazing as it once was. Soups are now in cups not bowls, the alcoholic shakes at Sci Fi are served in juice glasses. Some "filets" are noticeably NOT filet mignon cuts anymore. The Chocolate Wave at Coral Reef no longer has ice cream or a chocolate wave decoration accompanying it, it just lava cake. They have taken off many of the more intriguing entrees like buffalo steaks or lobster tails.
I have never used the DP but I feel like it comes down to quality vs quantity and perceived value. The DP has a high perceived value because guests see how much they are "saving" when looking at the prices of the items ordered. However, the prices stayed the same or increased while portions shrunk, reducing the ACTUAL value of the items. Furthermore, quality has also been reduced in many cases as well, reducing value even more. This is more noticeable to people who don't use the DP and locals are dining at Disney less and less.
I think it also depends if you go to Disney for the rides or the atmosphere. Personally, I can have a great Disney vacation without going on one ride. My favorite things are walking down Main Street, watching fireworks, and exploring all the resorts, lounges and restaurants. Now, if I had kids, I would focus on the rides and attractions more and less on the lounges and restaurants, so perhaps the decline wouldn't be as noticeable to me because my focus would be different.