I'm not entirely sold on the 'if you can summon Genji at 20 minutes, your probably winning anyways.'
That would probably make Orin&Alice combo the most powerful characters since a destroyed tower gives about 40 faith. If you got 0 kills and kill two towers, you can summon Genji. Probably sooner if other members of get some kills. If I was Alice and Orin was in another lane, we could probably summon Genji as soon as 10 minutes, then you'd REALLY be able to do nothing against Genji.
Additionally, what if the other team has about the same amount of kills and destroyedf a tower, and you summon Genji. If both teams play tight, that typically means that the gate is STILL going to be destroyed because Genji at that stage is quite overpowered, even though late game he is a joke.