The next time you are about to extend a class, stop and think about who is going to be consuming this extended class. If the extended class (instead of the base class) is in the contract amongst collaborators, why the extension?

If, somewhere along the line, you want your extended class to be used (consumed) as the base class, go ahead and extend. If that isn’t the case, then maybe composition should be preferred.

It really comes down to this:

Extension is about consumption.