"Scumbag" is a very gross metaphor....the term refers to a used condom.

Hmmm, not over this side of the Pond, Bill. It just means a bag of scum, i.e. a "scummy" low-life person with no redeeming features whatsoever. It probably implies a bit of physical dirtiness as well, along the same lines as "scuzzy".

"Nobody has been scumbag rockers like us and lived to tell the tale" is a proud statement, because it's a statement of survival against the odds. The Stones have been that low, but don't necessarily see themselves as low-down "scumbags" now.

I like George Melly's (alleged) line in response to Mick Jagger's calling his wrinkles "laughter lines":
"Surely nothing could be that funny".