Hi - I'm an A Sol main and also happen to be on the balance team. :)
Aurelion Sol is a pretty unique case of a champion. Before his mini-rework, Aurelion's 'depth' ( defined here: https://nexus.leagueoflegends.com/en-us/2017/11/champ-popularity-mixing-math-art/) was low enough that his high WR (generally around 53-55%) was more likely fueled by people just trying him and dropping him, than one tricks maining him. In other words, he was just strong and people who were trying out were winning a lot...because he was actually a 55% WR champ. Post-rework the amount of people maining him as gone up a bit, enough so that those one tricks may be influencing his winrate a bit more than before. Regardless, per our balance guidelines (posted here: https://nexus.leagueoflegends.com/en-us/2019/05/dev-champion-balance-framework/), Aurelion was OP and needed a love tap.
We generally do not approach balance with a philosophy that accounts for the amount of mains playing a champion when assessing what their strength is. If we balanced using this philosophy, we'd have to concede that Riven and Yasuo are actually underpowered and in need of big buffs. The reason being is that Riven and Yasuo are champions with high breadth (they're popular like you said) but also insanely high depth (how many games a player puts on a champ in a given time). The situation would, in reality, be the opposite of what you're saying. Everybody plays Riven/Yasuo and they also all main Riven/Yasuo - infact the ratio is so steep that they are the champs that should be sporting 55% WRs.