Custom software is a business risk not an asset.  Your company processes and people become dependent on workflows that lock you into the software.  You become beholden to a single vendor, and eventually the pain of switching away from a woefully outdated system becomes all but impossible.  Just ask the companies that still rely on hundreds of millions of lines of COBOL.<p>To add to this:  Very old custom software located on internal systems is one thing, imagine if you are relying on 15 year old <i>web-based</i> software from a vendor that no longer exists.  How do you protect that?