That’s a very interesting question and I’m not sure of the answer.
Obviously on some level, the cost of the infrastructure scales with the number of people using it. But so does the ability to crowdfund, if there are 100x more instances then theoretically there would be 100x more potential donors to meet the cost.
One clear way to influence the scaling in our favor would be to utilize instances with clear themes and purposes. If everybody on a particular instance is interested in the same content, that reduces the wasted computational resources compared to an instance where all of the users are interested in different topics, and thus subscribed to a much wider variety of communities.
My intuition is that as long as the platform only hosts text and images, the costs should be manageable, especially with inevitable improvements to computational efficiency that are likely to come as Lemmy matures. For instance, I believe there is some kind of patch that reduces storage utilization that should be shipping with the next version (0.19).