I get that. Animated films are usually among my favorites, which shouldn't be a surprise coming from me lol.
Ice Age is basically the textbook definition of Sequelitis. It started off strong, and would've been great as a standalone film as you said. When the company saw what they had, they went for a few more. Two and three had different themes and atmospheres compared to the first film, but they still worked for what they were. Even some big name critics positively compared three to the first film, some even calling it the best out of the trilogy.
Four hammed it up too much, and five jumped the shark several times over the course of a single film. Let's hope it stops there.