I would suspect this is because when 99% of users hear of an upgraded gmail service that costs money, they will start asking questions as to what's wrong with t

I would suspect this is because when 99% of users hear of an upgraded gmail service that costs money, they will start asking questions as to what's wrong with their current gmail service. They get so much synergy across products from users data it's probably not worth it to point out current gmail drawbacks to 100% of customers: even if x% switch to premium, 1-x are now aware of drawbacks of using google across all products and could unpredictably hurt top line rev.