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.