Great point. A good rule of thumb in jobs is that if you are the undisputed top <X>, make sure that you are being compensated very, very well. Because you just sold out. It is quite hard to reach new levels of expertise without explicit, task-related mentoring, and if you're trading off that kind of growth, do it explicitly.

Back when I was a manager, I used to find it somewhat depressing to interview people who had gone to startups or non-tech firms for 5-10 years, reached "senior developer" or "architect" roles, and whose levels of expertise were indistinguishable from the median recent stanford grad. Certainly, they knew more "things" but had failed to grow in either scope or depth.