No, development is not a running cost. It's a development cost, that may be sponsored (as was done with Keet), or paid as an investment by a company that does plan to make revenue from a P2P app (with still zero running costs), or donated by individual contributors.

The difference between development costs (even ongoing ones) and running costs is that running costs scale with number of users on the platform. If you have 4 billion users your running costs are substantial on a server based system. They are still zero on a P2P platform.