Your question seems to be based on a HUGE misconception about Objectivists ethics:
Objectivism does NOT say that the theft is immoral because it prevents the millionaire from living qua man. It is immoral for the thief to steal because it is incompatible with the thief's life qua man. The reason for that is that stealing is antithetical to life qua man--because life qua man means surviving by means of your own rational faculty.
The millionaire's life is solely the millionaire's business; it has no direct relevance as far as the morality of my actions is concerned. Objectivist ethics i