I think maybe it's a combination of things that have changed Disney restaurants over the years. While I'm sure the increased popularity of the dining plans has contributed a certain amount to this, I think that like other similar types of locations (other theme parks, stadiums, movie theaters, etc.), WDW had a captive audience and thus could get away with charging higher prices than a similar restaurant off property would be able to, and still keep the restaurants full. Plus, we have noticed in our area (NYC) as well as others when we venture out and travel, it's either a decrease in portion sizes and types of offerings on menus we have noticed, or an increase in price. This has been over the last 5 years or so, and we have noticed it pretty much everywhere we go, not just WDW.
Magical Express and Disney transportation have enticed more people to stay on property, giving Disney even more of a captive audience (people do not have to rent a car if they stay on property, and there is so much to do that they can spend a whole vacation without venturing to off property Orlando attractions).
Either way, we still enjoy the food at WDW! We don't get any of the dining plans (we use the TIW card) and we still try to eat at as many Disney restaurants as possible. And if we couldn't get the TIW card and didn't do the dining plans (none ever fit into our restaurant plans), we would still do our trips the same as we do now. It's a matter of preference and we prefer to eat on property.