This isn't a good thing, it's an arbitrary lock. It doesn't add anything, it doesn't make anything easier. Splatoon is a shared experience, adding content over time makes more sense. It's not at all comparable the Animal Crossing, which creates a setting which changes over time and has unique events for the player to experience. Mario Maker isn't a setting, it doesn't need real time events to make it feel more real or create a sense of progression. It's not comparable to fighting games that have you unlock new characters or stages by playing the game, there's a key difference between content unlocked by skill and content unlocked by time.
If new content was unlocked by playing the game this wouldn't really be a huge issue, people can learn the ropes with basic content before, unlocking new content and moving on to that, without needing to wait on an arbitrary time limit. The it's only 9 days argument just doesn't excuse it, explain how making an arbitrary lock not that long, (but still pretty long) makes the situation any better. If I died before I unlocked all the content it would be an arbitrary lock, if I didn't and then went on to live to 120 years, it would still be an arbitrary lock.