When you ask a question like that on this website, you're bound to get a positive answer. However, that doesn't really answer the question whether you "should".

Looking at the Steam discussion I see you're aware of the numbers and while there definitely are cases where Linux sales have exceeded their market share, it is fairly unpredictable. However, the conclusion you made that most Linux+Mac users also have Windows is not true. You can check the stats on this site to see that the majority don't dual boot.

I suppose the question is whether you would want to support it. The desire to have their game available on as many platforms as they can manage is usually the first reason for smaller developers. The number of direct responses on the forums will never lead to a justification on its own.

As for support. Since you used Linux for development, you should have a reasonable idea of the problems you can expect. But except for the users of "WeirdDistro_Fork14_Modification12", nobody would expect you to support that particular one.