If you have hero X and they have hero Y and hero X is so countered by hero Y that you’re probably going to lose, that’s not healthy for the game. How many times have you been in a game and you have a Widow one trick (pick any hero) and they just get dived by Winston/DVA/Genji all game and you lose? This is an extreme example where the combination of 3 heros counter a character.
Now think about Winston vs Genji or McCree vs Pharah. The existence of Winston or McCree doesn’t mean Genji or Pharah really NEED to swap. Not think of Brig and Tracer. The mere existence of Brig means Tracer probably has to swap. The burst heal on dive targets and armor means Tracer is going to have a hard time even if Brig does ZERO damage to Tracer.
This game is too reliant on teamwork that when HARD counters exist, you just get a big fat L whenever you get a one trick on your team and it’s not a fun way to lose.