I can find no data but I have long hypothesized that the rise of Starbuck's coincides nicely with the fall of Baskin Robbins. I think this gets missed by nearly everyone: coffee is an agricultural crop subject to delicate chemical processing that then decreases in quality over time. Starbuck's spends a lot more effort on supply chain consistency than Dunkin' Donuts does, and hella more than Yuban (for example). All the bullshit people do to coffee beyond brewing it has more to do with the bullshit than the coffee which is why most Starbuck's orders are "coffee with seven modifiers" - supply chain consistency or no, you're absolutely right: "What they push is blends that have little distinctive flavor." I think this: Comes from a consumer inability to acknowledge that the majority of the flavor they experience comes from a chain they know about as well as they know the winemakers of the Zinfandel they drink (and red wines are orders of magnitude more stable than coffee).Starbucks doesn't sell a lot of straight drip coffee. They sell a lot of milkshakes with a bit of coffee in it.
Most coffee is mediocre because making good coffee is hard work. I do very little of that hard work, my roaster does a lot of it. He tries a shit ton of coffee, he has two buyers who try more at different ports. Most my coffee comes from one family, farm or co-op. Each coffee is a unique product of how and where it was grown and processed.
I don't like their coffee but when I think about how hard it must be to make that much coffee taste the same everywhere I have to admire them. I like McDonalds coffee better than Starbucks. It's blend of south American arabica's and they don't burn the living shit out of it. It isn't great but it isn't all that bad in a pinch. It's better than the coffee at the fancy hotel I stayed at this weekend for sure.
It's pretty difficult to ensure consistency in a product that is being supplied by numerous farms across the world, roast and ground on numerous machines with potentially different manufacturers and certainly different serials across the United States in house and/or by co-manufacturers, and then drank by millions of people worldwide. Oh also having to take into aged inventory/product shelf life, blah blah blah. The industrial coffee supply chain is an amazing thing. I always want to contribute more than I do to these discussions.