It's been really interesting to see the approach Nintendo has taken to the battle royale genre. Games like this and Tetris 99 are so simple in their approach, yet insidious in their design. They dig their talons and and just don't let go.
Nintendo's track record when it comes to anything online related has never been great. I'm not sure what would drive them to make this a limited time offering, unless that's just when their assuming the drop-off in play will happen, but even then they could just let it run its course and then shut it down later. But that's not how they've ever operated. Nintendo loves to hold its properties close, and doles them out at weird intervals.