Even though trash counts towards a user's quota... users are, by definition, using less than their quota. And the percentage of users paying for increased Drive

Even though trash counts towards a user's quota... users are, by definition, using less than their quota. And the percentage of users paying for increased Drive storage is likely a small percentage of overall users. So deleting trashed items will mean Google is storing a lot less data, and probably not impacting people's storage purchase decisions much/at all. I expect it'd be a net benefit for their bottom line.