We already have an oligarchy. They control the government, and can get laws passed the poors cannot.

What purpose should they serve, ideally? - focus just on investment and profit - improve society ie noblesse oblige - compete for public honor, ie holding high office

I'm not suggesting their is one answer, but given the rich hold such sway over government, I definitely don't think they can sit back and enjoy their high tariff burgundy while we have riots in the streets, first from the left and now the right.