Sixx is a douche. I remember several years ago, someone from their production crew told me that Nikki could raise his hands in the air, and the "bass" would keep on playing. 
He plays along to it, but he plays. Live, Sixx has a sub-octave synth bass running parallel to his bass lines, I've seen and heard it at a Crüe gig in Cologne in 2009 to which my then teenage son dragged me (in fairness: it was a good gig, the Crüe can entertain in a dumb, but feel-good way). His combined bass sound at that gig made Geezer Butler sound thin - it is MIGHTY - and it took me a while to realize that there was a deeper octave signal running underneath anything he played. Whether he triggers that with his own bass playing or whether it is sequenced with Lee's drumming, I don't know. Sixx himself is on record for saying that the Crüe have been using that sub-bass enhancement/track since the late 80ies. He has no issues admitting this. (I have yet to hear ZZ Top admit that they use tapes in their live performance on all the Eliminator era stuff.)
It was also clear at that gig I saw that the background vocals were all sampled - the background vocal capabilities of the Crüe guys wouldn't even allow them a job as a stage tech at a Beach Boys concert.
Finally, there is Mars: Already in 2009 he needed a contraption that supported him and helped him to even stand. His guitar playing seemed fine though, he's not a bad guitarist at all and often even quite tasteful if one may use that word in connection with a Crüe performance. But given how physical the band's live act is and how Mars' special arthritis condition keeps deteriorating, it couldn't go on forever like this. Similar to Judas Priest's Glenn Tipton and his Parkinson's Disease that has curtailed his ability to perform a full gig, it was a matter of when not if.
