You're deliberately considering the third match of a bo3 as a bo1 for the sake of your argument, completely forgetting that the chances to adapt-in series are not the same in bo3 and bo2. Say team A is facing team B, so they both bring out their strategies prepared from scrims, A and B respectively.
Team A's strategy was superior and earned them the win over team B, however now team B can adapt, develops a counter to team A's strategy and takes the second win.
End in a bo2. Teams A and B are in equal in skill. Truly a >better way of ranking teams in relative skill . Why would you want to rate teams in relative skills? in relation to what? to each other?
Seeing the infinite number of variables that influence a person's skills it is highly unlikely they are truly equal, as you suggest. Truth is, most probably there is one team that is better than the other. So let's continue. Now team A knows team B's counter strategy and can plan accordingly going into game 3. This game is what you falsely call a bo1, ignoring that both teams now have information about what their opponents want to play or cannot play, the strategy they have prepared and how to counter it. All of which was not considered and not known in the first game of a bo3, bo2 and a bo1.
So now team A knows what champions they can take away from team B and do so, managing to pull their strategy over theirs and have complete control of the game. Until they throw at baron and lose. According to this bo3, team B is better than team A. Which might be untrue, but even so, team A had an extra chance to adapt, which you're unfairly dismissing.