I don't even see an option to to set a default calendar in Settings, it's just not on the list. I can change the digital assistant or a browser, but not Calendar. You sure it's a thing?
As long as an app registers itself as a handler for ics files and calendar intents, you'll see the app picker when you get one of those. There is no notion of calendars in the OS. Apps just register with the OS what kinds of intents and URIs they can handle.
They get stored in a system table and you add accounts this way as well, right? This is also how DavX provides CalDAV support: it downloads the calendar and syncs it with the system tables.
In any case, at no point was I presented with an option to choose a default Calendar app or nowhere do I see a way to change that.
You would only get presented with an option when an Intent has multiple possible handlers. If you install a new app that handles the insert event intent, then you'll get prompted the next time that intent is started. I just verified this by opening an ics file after installing a new calendar app.
