Wrong, he was a millionaire throughout his career, the highest ever for tennis at the time. He had sponsors out the arse, including a huge deal with Dunlop tennis racquets, and many more during his playing days. He was the biggest name in tennis, hands down. He was the "bad boy" of tennis.
And his fortunes continued after he retired with being a highly paid commentator as well as still doing endorsement deals. Oh, sure, he calmed down after he retired, nobody said he was maniac off the court, only when he competed. Sound familiar ?
And how did the tennis world treat McEnroe. His net worth is over $100M today. Yeah, I bet he feels horrible that he blew up on the court, won more championships than anyone else, and earned more money than every pool player that ever played, COMBINED.