If you think RHCP and Jane's Addiction are anywhere near their prime,then you have a shitty taste in music.
So bands have to be "in their prime" to be a headliner? Paul McCartney is nowhere near his prime but I think everyone would agree that he IS a headliner. The same could be said for Neil Young. Jeez you've got to be a piss poor person to hang out with when it comes to music. I saw the RHCP at ACL this year, and yes, they put on a GREAT show. It was way better than I could have expected and definitely worthy of a headliner spot.
That being said, these types of acts have always been and will always be headliners. So I'm not sure what point you are trying to make about them.