I have some questions: since Bitcoin proved itself to be a way better investment than basically anything else, why would anybody basically sell their coins for anything else, unless diversifying, of course.

Tax avoidance? Sorry, that's weak. If unsure, one can mix his coins, send them to fresh new own addresses and keep them safe.
So I will not even discuss about the trust issues your deal comes with, there's no need. You offer is not a good deal for bitcoiners. That my sincere opinion, but it's not a financial advice (one has to decide for himself).