Now that is a bleeding generalization. As long as Feral is in the Linux market we know that Linux ports can be profitable. There have also been developers that have provided sales numbers for their Linux ports and for many of them porting has been a success.

Maybe that won't be the case for all games, but low market share does not tell you much. Remember that the platform-specific cost - even with rewriting the renderer - of a game will in most cases be dwarfed by design, voice acting, localisation, marketing, etc which all are already covered by the existing target platforms.

Your assertion is also not well defined since "worthwile" could mean profitable (which will be the case for third-party porting companies) or more profitable than other development efforts.