Playing on harder modes limits your abilities and resources, but it doesn't do jack shit to the AI except make them hit you harder.
Also, people talking about the game being two years old forget it's now packed in with every PS4 and that console sells more than either of the other two, and apparently sells to a lot of people who did not own a PS3 last gen. So it will continue to be a new experience for many people.
edit, the DLC Grounded difficulty might have had some actual AI changes, maybe not. I wouldn't know, I was long done with the game by the time it came out. But I played half on Normal, found it boring so beat it on Hard, then got about halfway through again on Survivor or whatever the last difficulty was and was bored again and noticed no AI changes at all except how hard they hit.
