I can understand some restrictions like upload sizes (although even for unlimited subscribers they are way too small to be useful), but at least have unlimited features for Proton's own tools like Sheets and Docs. That would further incentivize users to use them as well.

The 10 MB limit is likely an outdated restriction that doesn't reflect Proton's current technology. Since Proton already has a secure storage system (Drive) and collaboration tools (Docs/Sheets), the technical challenge isn't processing files, but integrating them directly into the chat. In a "Unlimited" tier, Lumo could read and analyze files directly from Drive without physical uploads, effectively bypassing the size limit entirely and making the service truly useful.