I can understand paying $20 a year for a subscription service with extra benefits, but locking online behind a paywall on top of that is very much an insult to all Switch owners.
I don't have an issue with paid online as a concept. It's understandable that companies can't provide online for free, and have servers to maintain. What I do have an issue with is Nintendo charging a subscription fee when they don't even have dedicated multiplayer servers for their games. Splatoon 2, Mario Kart 8, etc, all use Peer-to-Peer for online services. This means that, beyond a very small server acknowledgement, all network traffic goes between you and the other players. This is why Splatoon 2 is particularly unstable. Charging for a service that will seemingly never see proper online multiplayer is an insult.
In short, paid online with dedicated servers is fine. Paid online that is unstable, without dedicated servers, which is effectively monetizing network bandwidth that Nintendo doesn't own, is not.