The reason you probably haven't noticed any difference is 2-fold: 1) the biggest changes probably started to occur when you were no more than 5 years old; 2) most fast-food chains when they make changes generally do it gradually over years so the public won't notice, particularly when it comes to cutting quality items in their product, as opposed to launching a new product, such as a different crust or new topping.
I just realized what happened back in the early 1980's and why I thought Pizza Hut's quality began to change at that time - Frank Carney, one of the creators and founders of Pizza Hut in Wichita, sold his ownership to Pepsico at the time and left the business. It appears to me that once Frank was out, the corporate big wigs went in to cost-cutting mode to increase the profits and the stock value. The same thing happened to Godfather's, but their changes were more drastic when Pillsbury bought that business.
As far as chains go these days, my wife and I will occasionally get a Papa John's pizza, as it is the only one we can more than tolerate. We like their original crust - the thin is not all that great IMO. Other than that, it's pretty much local pizza establishments that we frequent when we order out.
I agree Donato's is good for a chain pizza, but as you pointed out, they are a regional chain and not available across the country. Yum! Brands has the monetary clout to market and blanket their product across the country, and they know who their biggest audience is: 13-year-old kids who think General Tso's chicken is an exotic, spicey dish.