I've abandoned self-labelling myself a grognard. Yes, there are things I grumble about. More importantly, I know why I do and don't like certain games or certain aspects thereof. At the same time, I have no problem with someone liking a game that holds no interest to me.
At worst, if asked to participate in a game I have no interest in, I think I'd give a polite refusal along the lines of "thank you for the invitation, have fun." It might include some more explanation like "x doesn't appeal to me".