Bitcoin will not destroy central banks anymore than gold or penny stocks have destroyed central banks.

Lets not forget that governments control the gateways into and out of bitcoin and if bitcoin was a real threat they would have choked it to death already.

If bitcoin is the future then central banks will simply create their own version of bitcoin (ie a national digital currency with limited supply) or use bitcoin itself to back their currency (perhaps this is what smaller nations will do).

I don't think central banks are evil but they do tend to take the path of least resistance unfortunately.