100GB of data storage doesn't cost us that much money, since we're based on object storage.
You're right that the word "forever" implies outlasting the life of our company. But while we're around and existing, we will not take away the free plan from anyone who has it, and we intend to keep it around for new users.
It's not just that we're raging philanthropists. We want to make money, but our business model is to make money off the larger use-cases, not take $100 / month from people with a credit card.