Just like you include the pick rate for nerfing, it should also be used as a buff metric. Right now, you determine buffs in iron-diamond III based on win rate exclusively (<49.5%). However, Dr. Mundo has 1.28% pick rate in SoloQ with 49.5% win rate, so he doesn't meet the criteria for getting buffed. This is so wrong because not only is he picked <7.5% in diamond II-pro, he's had ~0% pick rate in this elo range, meaning that he's far and beyond unviable (useless) in the highest levels of play.
In other words, if he had higher pick rate, he'd most definitely have lower win rate (much lower than 49%), which means that he is deserving of buffs (QoL changes). Therefore you need to include pick rate in amateur play too, for example, if a champion has <2% pick rate.
Not including iron-diamond III pick rate as a buff metric is ridiculous, champs can maintain 50% win rate with 0.01% pick rate, that doesn't mean they are perfectly fine.
TL;DR: Pick rate, even in aver...