Right, so Peer from IGN brought up a really good point in this week's Nintendo Voice Chat podcast. Basically, he suggests that Nintendo may be trying to do something similar to what they did on the Super Famicom with the Satellaview. That was basically another subscription service Japanese players payed for in order to have these kind of special event games. It's really difficult to explain, so watch the link. It's a pretty novel idea.
The thing to focus on though is the part on the Switch's online page that says that the monthly NES/SNES games have added online multiplayer. Peer suggests that this is actually the point. For a modern example of this, look at the Splatoon Splatfests. It's a scheduled online multiplayer event meant to get players to play one specific game at the same time, and people love those. If you look at the online games from that POV, it's a completely different beast entirely to a traditional subscription model, because the point isn't that you're getting free games. The point is that you're participating in a timed community event.
So let's say that Super Mario Kart is the game of the month. The point isn't that Nintendo is giving you Super Mario Kart for a month and then taking it way - the point is that, for that month, everyone will be playing Super Mario Kart together, which is a really novel experience. And as soon as the novelty for an online Super Mario Kart wears off, Contra III will replace it, and then Mortal Kombat, and then TNMT, etc. That's a really cool idea if you ask me, especially if the subscription really is only $20-$30. It's unique, but in a way that's actually pretty neat.
It also reopens the pipe dream of a virtual yearly subscription service, which becomes more enticing the more I think about it.