How can a Linux client pay the bill if it is available as a separate, free-of-charge, download only?
Sure, that's difficult to tell how many people bought the windows version and used the Linux client.

It's a great move from id software to open-source old game engines.
Apart from this - honestly, who still cares about id?
They didn't manage to release a real blockbuster since Q3A (D3 was a technology demo).
It's definitely not due to the Linux users that id is on the skids.