Overwatch will not steal LoL Players
The games aren't even relevant or similar. Overwatch and League don't compete for the same niche in the gaming world, so Overwatch wouldn't poach League players any more than any other class based shooter game. If it was a new MOBA, then yes, it might poach LOL players, but Heroes of the Storm didn't do that. Overwatch is most likely to draw in players who play Team Fortress, Star Wars Battlefront, or Call of Duty. I don't think there's any reason why an FPS game that's predominantly console based will be stealing players from a PC exclusive strategy game.