You assume everyone always chooses the option of highest return, however not everything in life is about opportunity cost. I personally know two people running

You assume everyone always chooses the option of highest return, however not everything in life is about opportunity cost. I personally know two people running test stake nodes. They've spent the time setting them up, documenting, and really getting to know all the ins and outs of it because they find it interesting. They will run production nodes. You only need some X amount of people to do the same to avoid overt centralization. Then it just comes down to an argument of what is the minimum number for X where things feel properly decentralized.