Overwatch is a successful game, but to what degree nobody knows because Blizzard won't divulge total units sold across PS4/X1/PC. Another thing that ALOT of people don't realize is that South Korea, Overwatch's biggest market - dont even buy digital/physical copies because players in SK gain access to Overwatch from Internet Cafes and pay for an allotted time for the service. So with that said - South Korea's player base for Overwatch simply CAN'T be attributed to sales numbers.
The Overwatch League however, will not be around for more than a few more years. Ratings and viewership have already peaked, with no inclination that they'll ever go back up. Blizzard still has alot to learn before Overwatch can truly succeed as a global esport.
I give it around 2021 when Blizzard cuts the cord on OWL.