Ah, I see. But it is pretty trivial to write modules to support new input formats. For example FB2 and EPUB as input formats were both written by 3rd parties. And CBZ/CBR was added by me based on the python code from comiclrf in half a day.
As for an output format, given that it has to eventually be integrated into the GUI with its own set of options, it's probably easier to have a 3rd party write a module rather than a plugin. IN the case of the plugin, the 3rd party would have to write a GUI for the configuration as well, whereas with a module, I do that for free
Also an output plugin would have to satisfy a pretty rigorous set of requirements, so I would be more comfortable with it being developed in-tree.
Finally, from an end user perspective, having all plugins automatically installed with calibre is much friendlier.