That’s not how cost/user works. The cost/user actually goes UP the more small instances you have as a result of more expensive, smaller scale, and severely less optimized infrastructure. Infrastructure gets cheaper on a per-user basis as it consolidates, there are lots of technical reasons for this, but it can be summed up with scale (infra per “unit” is cheaper the more you can guarantee you’ll use, and LOTS of cost optimization paths open up the larger you get).
My point is that the community is going to hit a growth barrier, and that barrier is money and efficiency. Would you be willing to donate $5/m to 50-100 instances? Since to support that kind of scale they would need to whittle down to one instance per community for large communities, and massive communities (think 10-50 million users) might not even be able to exist with the current Lemmy hosting model. I wonder if even 1-5million user communities would even function without dedicated engineering to support the infrastructure and custom tools/services to make it work.
…etc
It’s a real problem. One that will be felt sooner than you might think, and one that will limit the growth, stability, and longevity of communities.