You don't get it. 1 key ability for a bronze. Not many of the pre-set teams and players have just one ability. It renders the player custimization pretty much useless.
You are wrong on this issue because there's no gurantee that it would have overpowered teams during online play since most of the pre-set players have 3 or more key abilities. Second the limited factor is the 11 players you choose not the 5 abilities that each player has. It's hard to get a dominate team with just 11 players.
In theory you could load a team with 11 created players all with 5 key abilities and put them on offense. Your defense would be so weak that the game would be 55-54. Legends are not gamebreakers but they are really good.
So far in the gameplay there's not much evidence that a 5 star bronze or silver player is a robo-player. They are good but not so good that it causes an imbalance in the game. You wrongly assume it creates a god-like player but this isn't the case because the limiting factor is the gold/silver/bronze levels and the 11 player selection.