To justify what price ? It's 20 eur/year... With that price you can't expect much more that stable servers for online, not even sure that 20 eur/year will cover efficiently server managing costs.
I'm not a big online gamer, and most games I play online are PC games, so wait and see. If they offer more interesting things, why not. But for now I'll pass.
Nah, 20€/year would cover server costs even if they would run the servers for Europe in expensive Switzerland, by far. Yes, 20€/year isn't much money, but it's expensive for what they offer. Server costs on PC get calculated directly in the one-time buying price. I admit that most first party Nintendo games with significant online part probably will be longer active played by many people then the average PC game (with the exception of ESL games like Overwatch and Counter-Strike and such of course), but making e.g. the port of Mario Kart 8 full price (yes, yes with all DLC but still) and then asking for a monthly/yearly fee for online play is straight-up cheeky. Nintendo not even looses money with every sold console unlike Sony and Microsoft, there really is no need for them to overprice this service to get some money back they lost somewhere else...
Is it alot of money? No. Is it fair priced? No, not if they don't add more of value to the services. I totaly understand that you probably think "If overpriced or not it's just 20 bucks a year, it's really not worth complaining about that

", but for me it's about fairness and customer friendliness.