Hot take: American fast food chains haven't gotten worse. Actually, the quality of their food and the atmosphere have generally gotten significantly better in the past decade or so.
The difference is that now you are an adult, and you aren't as enthralled by a cheap, bland cheeseburger as you were when you were 8. Your fond memories of going to the fast food joint with your friends in high school are fond because you are remembering formative memories, not because the food was actually good. You enjoyed stopping at the fast food restaurant as a young parent because it provided a brief respite from the demands of taking care of a young child. You think Taco Bell tastes like garbage now not because Taco Bell has gotten worse, but because Chipotle has raised your expectations for what food should taste like.
Honestly, the trend of pining for the fast food days gone by feels pretty cringe to me. Like, c'mon, are you really gonna look at me with a straight face and tell me that McDonalds in the 80s really cared deeply about making children happy? That Taco Bell was respecting their customers' high culinary expectations when they introduced Mountain Dew Baja Blast and the Doritos Locos Taco? That back in the day when the staff at TGI Fridays came out and sang happy birthday to you, they really meant it?
No!!! None of that was ever true, lol. These are giant, soulless corporations completely geared towards maximizing profits, run by stressed out managers and pissed off employees, and they always have been.
What actually happened is that Americans got higher standards. They became more knowledgeable and worldly and adventurous. Their expectations for the taste and health of the food they ate grew. 20 years ago in middle america, few people had ever eaten Thai or Indian food. Sushi was exotic and dangerous. Eating a salad for lunch made you a health nut at best, but more likely identified you as "probably a homo". Olive Garden was actually considered a good place to take a date. We changed. We got better. This is a good thing.