The problem was the last game. It kind of ruined the entire series. I didn't like the artstyle change between games. But it also just felt like it was not as good compared to the previous entries. Also they introduced time travel, time travel is like a story killer. Of course, the story was already dead with other choices they had made.
I felt like a 4th sly cooper game should have been about Sly's child since the themes were already about some multigenerational thief family. Sly's child would have wanted to be a cop just like mom, and then been confused after finding out that his family's criminal roots. It could have been a more adult story, for the now more adult players who were children with the previous games. I dunno.