My understanding is they are focused on user growth and improving the UX right now. Also, I believe they could be sustainable without being as widely popular as dropbox. IIRC eventually they will have a paid tier for their kbfs solution. I know I would certainly pay. For me Keybase is a one stop shop: identity management, individual and team chat, encrypted git repos, and secure file sharing all on a cross-platform system. No one really offers the service they do.