Thank you for giving us your experience in this.
We know this happens from the trials. Its likely there will be some patients who need to be treated indefinitely. However I think they expect that repeat treatment will increase the actual cure rate. In the meantime we are really talking about temporary remission for the majority. Its long lived plasma cells that are the hypothesized problem. Rituximab does not target them directly, and they can stick around for quite a while. If even some that cause ME, in the autoimmune or immune versions of the hypothesis, then it comes back. If a treatment gets rid of them though, then they are unlikely to come back .... with the caveat that since we are still not sure what causes ME this is only hypothetical.
The cure for MS might also work with us. This was discussed here a few years back, and there is a new thread on it. Other avenues are still being tested. Some of the newer (and more expensive) drugs might do much better, but I think cost is one reason why research is lagging. However a brand new drug, right off pharma's new product line, might in theory get pharma support, and that could fund trials. The downside is the cost of that kind of drug to patients or insurers or government would be huge.
Nonresponders are a separate issue.