I too have always thought that Starbucks is over roasted. IMO everything they make tastes burned. I have had dark and French roasts that are strong but not burned tasting. So I know it can be done.
Diners ......... well it varies a lot. Some good some OK, some terrible. I have not had any that was really good.
Just about every mini-mart, gas station, and pretty much every other type of place sells coffee in the USA. And most just do it to provide
it and have it available. Most I have had are not good. Sometimes OK.
I have had some truly outstanding coffee in better restaurants. So few though that I can pretty much remember when and where. Even in better restaurants it is generally just good.
Lots of simple factors though ..... cleanliness off equipment, the WATER going into it, staff that cares, etc.
I tried Tim Horton's once or twice (same location) and it was only OK. Seemed really weak. But even within a particular chain it can vary a lot. EX- I have two McDonald's near me in cities in opposite directions. One has good coffee consistently. The other has bad coffee consistently. I am pretty sure in that case it is the water. Because the same holds true for the water in restaurants in those two cities.
At least the coffee I am drinking right now from my own kitchen is pretty good