Ideally yes, but we have no idea what other new code might be added for entirely legitimate reasons. Or it could be a complex interaction with some other system that was triggered by a date. Just too many unknown variables.
I think that’s smart—BusyCal and FantastiCal are worlds better than Apple’s Calendar. However, they’re still getting the data from iCloud Calendar or Google Calendar or wherever. That’s usually fine, but it means they’ll show exactly the same data as Apple’s Calendar.