One of the issues that’s come up in the Old Mongo removal work is that certificates code depends on Modulestore to store instructor signature images to put onto certs. This means that we can’t remove Old Mongo file storage functionality without breaking certificates, and we have absolutely no intention of breaking certificates.
This hammers home an issue that repeatedly crops up–so much so that there’s an arch manifesto point for it: It’s generally better to copy the data you need for your particular context than to rely on a runtime dependency on another system.