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 You become beholden to a single vendor, and eventually the pain of switching away from a woefully outdated system becomes all but impossible.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.

To add to this: Very old custom software located on internal systems is one thing, imagine if you are relying on 15 year old web-based software from a vendor that no longer exists. How do you protect that?