Not just speaking technically but economically. As user number mount up their post is just going to add up. I know instaces may distribute user, but server space isn't going to go down for anyone. It just seems like built to fall.
i think the resources youre referring to are easily, fluidly handled and there should be no long term concern as strategies are developed to handle those resources. server space can be recycled, caches cleared, etc.
youre not wrong in that storage is a concern, but it is one of the cheaper components from my POV running a full instance.
I'm running an instance, on local home fiber that I'm already paying for (30€/m around 700Mb up for real, 2x2.5Gb down theoretical) so no problem today, at all.
Don't make the error thinking today's tech will linger, we will most probably have better and cheaper hardware in the future, but also smarter algorithms dealing with it all, remember it's a first try and can only get better.
i think the only sustainable model is having a donation process built into the site. its very successful in other platforms.
its that or ads. no one wants that.
my storage costs are currently < 1/10th of the total o f my other costs. sitting at $1.00/day on one of the most expensive providers. my server has 12 users