I too have noticed that some of the more popular restaurants have dropped the bundle on their food quality, they were great at one stage but their quality has dropped a lot, The thing is I have also found that local mum & pop shops have maintained their quality and service so we have dropped going to bigger name shops and go to the smaller local ones and still get great food. If I had to put up a reason it would be profits in the bigger name ones, they are now after a lot more profit and dropping quality and service are the result while mum & pop shops havent changed, they rely on regular customers without all the fancy bits & pieces, they are cheap and very good while maintaining the quality of their ingredients as well as service.
As for coffee I find starbuck s coffee is garbage as is most of the brand name coffee, again they rely on their name only and not the quality. If I want to get a good coffeee I go the the smaller shops, I select the roast not accept the style they make the shop use or I make it at home with beans I buy and grind it myself. Many thais as well as other only go to branded shops because they want people to think they are more than they really are, same with some restaurants, when I read about all the crap about big brand shops I cant stop laughing, this is Thailand, its not the west so everything depends on profits, this is one of the first things you learn living here, profits are more important than anything else especially with branded shops, maybe do as the locals do and eat at the better mom & pop shops