A lot of people mistake open source as some form of insurance. It can be but having the source doesn't mean you can just fork it. Especially when the original tree is under constant development. Bitrot can occur when you don't track from mainline particular features that are seen as essential. Bitrot also occurs when the codebase doesn't change fast enough to match its surrounding ecosystem.
If you're running a site and you open source you definitely need to make sure your core benefit transcends that source code. Your staff, your experience etc. Otherwise you are setting yourself up for basic competition from others using your technology.
None of this stops open source being useful or a good idea in general. Just make sure there's an actual benefit and you know exactly what that is and what it implies.
As mentioned before, I've had customers want the entire product open source but on further inspection I discovered they really just wanted the ability to take their data out. They really wanted interoperability.