Yeah, the code obviously remains under the BSD no matter where you move it - or else you wouldn't still have to follow the rule about the copyright notice.
But for all practical purposes, it is "swallowed up" by the GPL or proprietary licenses, because you can never get anything back out. Any changes to the code could be GPL or proprietary, and therefore not allowed to be copied back into the original upstream BSD codebase, and of course you already have the original code so whatever license that is under really makes no difference.
Yes you can never get anything back out from GPL, but that is nothing to do with the community. BSD is useful for closed source companies, and closing the gap between closed and open. We don't have any reason to produce anything for monetary companies, so we always want something back, and that's why we use GPL. I'm just saying that the monetary companies dreams for a BSD world, will never come true. The 95%+ of the free code, will always be under GPL until the end of patents and IP, and then the end of monetary systems.