When a WH starts to go, often, but not always, it starts as a slow leak. A drain pan can handle that; it cannot handle a major leak, but hopefully, you'd notice it before that happens. One thing that does work is to install a WAGS valve in the pan that will shut the water off once the pan starts to fill up. If you use the included switch, and the WH supports it, that can also shut down the WH so it doesn't try to heat what may be an empty WH. www.wagsvalve.com . It also gives you a free insurance policy IF it is installed by a certified installer. If those plumbers have a valid license, a note to the inspector may make them sing a different song.