The Sixth Sense and Unbreakable are both masterpieces in my eyes. I just rewatched Signs 2 days ago -- in my memory the film lived as good but significantly weaker than his first two features. No, it's the same pedigree in almost every aspect. The difference is that Signs is less ambitious and ultimately a simpler film, but Shyamalan brought the top of his game in it without a doubt (maybe the writing isn't as excellent but close).

I wouldn't quantify like "ten years have to pass" but I agree with you. Split is still great, though. Not on the level of Shyamalan's first 2-3 films but not that far behind. Now Glass is a different matter. I hated that so, so much.