I hear people talk like this, but I don't think it is actually true. Sure, fast food use to be half of a smaller joint, but now you are only paying 20-30% less at the fast food places. That ignores the fact that a lot of the cheap food is on the apps now. My Mcdonalds has had buy one get one Big Macs for about 2 years now. Even if I get that and a fry, I am looking at a $8 bill as opposed to a local joint that is going to charge $9 for their basic burger, no fries.
This doesn't even take into account the speed of the fast food places, which is much slower than it use to be, but still the fastest places in town. So yes, the days of a late night snack run to Taco Bell are over, but the restaurants still have a purpose. The purpose is for when you need some food right now, and not for a huge price.
Stock price is really just a present value of future expected earnings. Buying Coke for $100 is because you think the earnings of that share in the future is worth $100. So yes, if the company makes an announcement that it isn't as profitable, the price will go down, because buyers won't want to pay the same for an asset that is returns less than it was expedited to.
Yes, there are complications. Shorts, futures, non dividend yielding shares, and more make it more muddied. At the end of it though, the future expected earnings are what is being bought and sold.