Next, any investment of time into Mapnik maybe not so wise in long term. modRana is probably the only client which is using it and I would expect that, at some point, it will also switch to Mapbox GL. Which will leave us with the backend that nobody is using anymore.
Use of the server on Linux is non-existent (probably only myself) and I don't see much reason for users to stick with Mapnik if suddenly all devices support Mapbox GL.