How do you plan to incentivize running a node?
The node running cost could be very cheap ranging from $3-$5/month for minimal node. Each node could set how much disk space and bandwidth it is willing to use and whether to serve all communities or a selected few. In general, users and community owners will not need to deal with setting up their node as long as there is a public node running. In the beginning two-three public nodes would suffice. If it would be necessary to incentivize running a node, it could be done, but in this case I do not think it will be needed. After-all, the chat servers are handling messages, not transactions. The worst thing that can happen is that the chat log history gets deleted. But with two-three nodes this should be very unlikely.