I hope you manage to get in a lot of time playing that Miivers Smash Bros. level in the last few days to enjoy it while it lasts! I haven't touched my Switch in weeks because I've been making the most of the Tingle Bottle feature in Zelda Wind Waker on Wii U before it disappears forever
I really hope they do something like Miiverse with the Switch, but I'm not holding out much hope. I did some reading when I first found out about Miiverse ending, and I'm pretty sure I read that Nintendo said they saw no point continuing it because everyone uses external social media platforms such as Facebook and Twitter so much that they didn't see a need for their own social media platform.
I'm also cynical that Nintendo are getting rid of it because Miiverse is free, and they want everyone to move across to the Nintendo Switch Online feature that they are about to start charging money for in 2018, rather than people hanging around on Miiverse to talk to their friends for free there.
I would love if there was something like Miiverse included in the Switch paid service, but even if there was I don't think it would be as fun because one of the great things about Miiverse is the drawing aspect, but it is impossible to draw well using the Switch (I seriously don't know how some of those people posting detailed drawings to Splatoon 2 are doing that, they must have so much free time because it takes so long just to draw something basic using the joysticks).