Success and greatness are definitely distinct. Success is (to an extent) measurable. If they weren't, Pharrell Williams (pop song writer en masse) or someone like him, would currently be the greatest living composer.
Even Mozart was largely unappreciated in his time. His music was viewed as too complex, and frequently had mediocre receptions at performances. But hey, the living generation rarely seems to like its own composers, and prefer the ones who play it safe/boring and hark back to the past, whom history will no doubt forget. (Kalkbrenner anybody? He was a much more popular contemporary of Chopin's)