Right now it doesn't cost much to host. But soon I plan to create a proper plugin store (currently there are only a few plugins available to general public as you may see) and encourage people to develop and upload their own plugins, so I will have to scale up for all that. Regarding the backend, it's a node.js application.
Exactly! socket.io was there mostly to speed up prototyping. I plan to replace it with vanilla code soon, to make the frontend app even more lightweight.
Actualy there is such a mode already (without payments though). Through the server API it's possible to create a room as a webinar that behaves exactly like that.
Probably what I should do is to allow people to create webinars not just via API but also from the interface.