It's a matter of professionalism by the teams.
By this system the best (current in group stage) team will secure the #1 spot, and the 2nd. will go through bo3 and eventually win that. (show other teams more strats/positions on maps than necessarily.
Another thing you should think about is why would teams want to play each other bo3 in group stage when the bo3 and eventually bo5 in the grand final will happen later in the tournament.