That may be true and also something we considered before we allowed the first user to become a lifetime member. However, there is no cost to board a new lifetime member and for that reason we can simply stop issuing new lifetime membership if it won't be profitable long term for the forum.
Right now we need funding to keep the forum running, that's why it went out to be a good idea. No one here is driving a Ferrari nor is getting rich, all money goes into the development of the forum which can be challenging from time to time.