And if, over time, people learning how to play against him better has a larger impact than players learning to play as him better, then his winrate will drop and Riot will buff him instead of nerfing him, unless his ban rate is still very high or his pick rate is high enough that buffs would only unnecessarily crowd out other picks and result in a more stale game overall.
I don't think that whether something is overpowered is based on some ideal scenario. If a champion loses 100% of the time if you play perfectly against it and understand all of its weaknesses, what does that really matter if in real scenarios players are losing 60% of the time against said hypothetical champion? Who cares about the imaginary 0% winrate when the real average winrate aggregated from players is 60%? That champion will still be objectively overpowered according to the statistics until players catch up to problem-solve how to defeat it more reliably.