First off, I have no idea what these other reviewers are talking about with this "alternate universe" theory. The fact is this movies ties perfectly with the first one. All the same characters are in the movie from the first one and many references are made to Doc, so not sure how this thought came about.
This movie is essentially a version of cars where Lightning McQueen actually aged, meaning he is not the same age as in the first one. With age, especially in sports, comes new, and younger, talent. In this movie lighning McQueen is faced with this same challenge, new, younger, talent who are training in new, more advanced, ways. With this younger talent and better training methods (Lightning McQueen still trains like he did under Doc) Lightning is having a tough time competing against the new guys. He essentially has to make a choice, adapt with new training or retire. I won't go into the details of his choice as I don't want to spoil it for anyone.
However, this movie was very good and, I thought, a fitting end to the franchise. Can you imagine a Rocky movie where Rocky never ages and just keeps fighting and beating everyone? No, and that is not how the Rocky series ended. Rocky retired and helped the new up and coming fighter. That is somewhat what happened here.
I can only assume the others making the reviews wanted Lightning to be perpetually young and just keep racing and racing and beating everyone, movie after movie. The writers wisely chose not to follow that formula. Yes, I will agree, there is a bit more of a story here than just pure racing, and perhaps that is what people didn't like. However, that didn't make it a bad movie.
I hated Cars 2 but I did really like Cars 3. It's not a perfect movie but also isn't what people are making it out to be. Give it a chance and I think you will like it.