I suspect they don't lack depth so much as have depth in other technologies which aren't used. If they don't want to stay on the treadmill of building expertise

I suspect they don't lack depth so much as have depth in other technologies which aren't used. If they don't want to stay on the treadmill of building expertise in the next new thing, then they move up to jobs based on the wisdom they have accumulated instead. I have also seen a lot of people hired in at high pay due to their laser-like career focus and years of experience in just the right area, who turned out to embody the peter principle. They lacked the brains and creativity to do anything new and still get stuck when a new twist comes up. Meanwhile I know generalists who are great at figuring things out, and got bored when they had more or less mastered the technology so them move on to a new challenge.