I think a lot of people are so used to living in this world of endless huge franchises that they forget you can make two movies, or you can make 3 movies, and stop while it's hot rather than running it into the ground. Of course Jurassic Park is more limited than Avengers or Star Wars because its main stars are the environment, non-characters. Even other monster movies such as Godzilla and King Kong have much more room for sequels and continuing the story because those monsters are sentient. The dinosaurs in Jurassic Park are just wild animals. There is only so much you can do.
Nothing wrong with this being a hugely successful trilogy. We used to have trilogies, you know... a long time ago.