Speaking from experience in a more positive direction, there's also a lot to be gained from learning a second company's culture and tech stack. I spent several years at a FAANG, and my technical skills improved considerably when having to apply my existing experience to a new set of tools and environment. I'm not sure a third company would have quite a stark difference, but I'm sure I'm a better developer after 5+4 years than I would have been after just a straight 9.

I don't think it's necessary to move every year or two, but I'm not sure it's ideal to go too far in the other direction either.