I vaguely remember Google had to put their foot down to disallow such use, that's how popular those filesystems became.
One of the most powerful advantages of Gmail vs the competition is it's superbly powerful spam filtering and search features - I doubt it's worth losing that powerful image in order to get a few more users to pay up.
Just like the biggest advantage and product feature ProtonMail has is privacy and encryption - it would be a bad product decision to remove encryption from all free customers in order to get them to pay up.
The alternative is to offer a fully functional free* model that focuses on your strength, and charge customers for extra - extremely useful and important, but non product essential - features.
* Some would argue that Google's model is not free, as you are paying for it with your data, but that is a different discussion and outside the scope here.