Stated differently: Diminishing returns on time invested to expertise gained becomes unattractive at the highest expertise levels. A lot of time is required fo

Stated differently: Diminishing returns on time invested to expertise gained becomes unattractive at the highest expertise levels. A lot of time is required for a small growth in expertise (pushing to expand the frontier of knowledge in physics, for example). You can be an expert, but not the world class expert, with less time invested (before diminishing returns becomes prohibitive). I like to think of this as catching up to the state of the art, but not actively working to advance it (much, much harder). Do this in multiple fields and you may have a unique skillset or perspective that's valuable.