Propaganda by people trying to sell the same thing as "healthier" has convinced a lot of people that it is true. It helps that the idea of a large corporation selling sawdust as meat is incredibly believable though.
I don't eat a ton of fast food anymore, and when I do its usually McDonalds, but I have always enjoyed the Whopper. I had one a few months ago at an airport and tried the Impossible Whopper last week. It's close, but the beef version is still better in my opinion.
When I have a whooper most of the taste is the sauce/bun/toppings with the patty only being responsible for a small portion of the taste, which makes it perfect for a substitute. I've had impossible whoopers a few time, and while eating the whole thing, it's basically the exact same taste. If I eat a piece of the patty with nothing else then there's a noticeable difference though.
Replacing questionable meat with hype pseudo-meat is a straight win all around. I'm as carnivore as they come, but if I have to hit a Burger King, I'm just going to get an impossible whooper: if i'm going to be getting a shitty burger either way, I'll take the one that didn't require all the logistics around cows.
This KFC one is just the next step. Replace all the questionable meat with pseudo-meat that is (hopefully) more sustainable, and leave the "real" stuff in caviar territory where people actually care.