Charging for cloud saves without an alternate way to back-up your save data is a poor move, whichever way I look at it. If there was a way to back-up saves to USB or microSD then sure, charge away. But locking the only save back-up functionality behind a paywall is poor form.
I was most interested in Nintendo offering a subscription service for their retro games, but given I've played (several times) the NES games I'm interested in, I'm not particularly interested in this NES selection. I also think what we're looking at here is a stand-alone app - 'Nintendo Entertainment System - Nintendo Switch Online' seems to be the branding for it, and Nintendo's press response about Virtual Console mentioned the NES selection separately from eShop. I wouldn't be surprised if you end up with an app that functions similarly to the NES Mini's front-end.
However, the wider ramifications of this suggest to me that when Nintendo want to add a new system to the subscription, it's going to be a new app so we'll have 'SNES - Nintendo Switch Online' and 'N64/GBA (etc) Nintendo Switch Online' at some point in the future. I'd be more interested in those, and I'd really like to see Nintendo give people an option to buy the games they want to keep. Ultimately I don't see Nintendo allowing people access to multiple collections of games for just $20 per year. Given the value they place on their back catalogue and intellectual property, I suspect that, in future, any new systems will bump up the cost of your Nintendo Switch Online subscription. Not by much - maybe an additional $5 - but access to NES, SNES, N64, GB, GBA games plus online play, cloud saves etc seems too generous to me.
That being said, I'd pay more for a wider selection of games from different systems.