There's nothing wrong with a band you like becoming "mainstream" - especially in your example. If anything it suggests the taste of the public is adjusting and that can be a good thing.

Confusion arises when people talk about an artist "going mainstream", that is, purposely adjusting to the current popular style of music - and in most cases it means going softer and turning to a softer brand of music - which in most cases people would call "pop". This doesn't actually happen often and when it does it's usually terrible (at least to fans of the artists previous work), especially when said artist was particularly inaccessible before.

I don't have any particular issue with pop music. It's just usually boring and tasteless. Having found music that isn't boring and tasteless inside and outside of the mainstream - why would I subject myself to listening to this "empty" music that seems to satisfy the general public?

Without getting carried away on pop music - "Damn, too bad this got mainstream. I liked them better when they weren't famous" is a tremendously retarded comment. Regardless of its status, you either like it or you don't. When people start judging you on what you listen to - that's when you've got to learn to not give a fudge else you become a boring sod like the people you're being pressured by. Songs you like "becoming" mainstream, if anything, should make you happy.