Hari for an extension that allows setting custom default values for each individual notification+driver, I'd say 1 to 2 hours of work. It might conflict with other extensions that try hard-coding new default values though.
There are also some challenges with values already saved to the user preferences. I don't remember exactly how it's done, I think as soon a user touched a setting, it will be saved to the database and ignore the default. I don't remember if this impacts the value for other drivers of the same notification types.
There also other potential issues regarding translations and visibility. An extension is not required to register its notification type into the standard notification grid settings. It could be an invisible setting, or be controlled by a dedicated setting. There's no way to retrieve the user-friendly name of a notification from the admin panel since the notification grid is in the forum frontend, so the extension would likely have to hard-code every known setting translation, or use some convoluted way to retrieve the forum javascript source code and execute it somehow in the admin.
I'm no longer part of the core dev team so it's not for me to say, but I feel like this might be a bit too complex to have this in core, at least with the current system. Having this feature in core means any change or addition to the notification API would require changes to this feature as well, which adds overhead to maintenance. If we wanted to make this an official feature, we would probably need a more advanced API where each extension can provide a label and description for each notification type to be rendered in the admin panel. Creating this new API will likely take more than 2 hours of work and will also require updating all existing extensions to make use of the new features.
