Is there a fallback when a Twig extension is not enabled?
I can't think of a native way to do this, but Twig does have a hasExtension method, which you could make use of in a Craft plugin.  But if you wrote a plugin to utilize it (similar to this) obviously that wouldn't help if the plugin was disabled or uninstalled.

Alternatively, you can add this as a feature request to add to Craft natively. If you go that route, be sure and go into details on your specific use-case as I'm not sure I'm following it 100% here.