I was directly answering the man's single argument--something you rarely do. Yes, there are other reasons for considering the men named professionals (except for Marlowe), but there are also plenty of reasons for considering Shakespeare a professional, like names on title-pages, and, gasp, posthumous personal literary evidence. I was quite prepared to bring them up if your friend had brought them up. In fact, I did just now, when you brought them up.

--Bob G.