Well I have some theories about what "mainstream" really means.

Go to your local shopping mall, sit there for 120 minutes, and count how many women you see and how many of them are wearing pleated skirts.

Take that percentage and divide by 2.

That is the largest percentage of guys you will ever see wearing kilts.

It will be the guys who love the comfort, the freedom, and the comments from the ladies and don't care if they look "different".

Do you think that is a good formula and using that guide what percentage of guys will ever wear kilts? I don't think it will ever be mainstream, however if just one percent of guys wear them it could be called commonplace, right?