I'm ambivalent about it. While it's important to have fresh ideas and new games coming through to keep things interesting, there's also value in evolving what is already there without having to go right back to first draft. With a sequel, you know roughly what to expect, and that can be a good thing.
It's a worse problem when there are too many movie sequels, because narrative is necessarily more important in movies. A game might be narrative-focused, but it doesn't have to be, so a sequel that keeps the story and updates the gameplay is nothing to be ashamed of.