Look, on one hand, I feel for you: Nintendo’s digital refund policy is archaic and awful, and a decade behind just about everyone else (aside from maybe Sony, where it’s more like 5 years).
On the other hand, if you absolutely NEEDED online play, you should have done the research to confirm it. If you visit the game’s online page, it’s made pretty clear that cloud saving is the only online thing the game supports. Googling “New Super Mario Bros online multiplayer” also would’ve done it. If you’re able to post on here, you’re definitely tech literate enough to do that.
I do think its a mistake for you to sell your Switch over this, especially since you clearly enjoy the games Nintendo makes. Simply applying a little more forethought to your future purchases on it is probably all you really need to do.
That said...I’d be lying if I didn’t say this lack of any refund policy didn’t adversely affect how I play and enjoy games on Switch. I agonize over most any purchase because I know once I do it, it’s mine forever. And on a service where demos are also uncommon, that makes it doubly frustrating. So if Nintendo’s content simply isn’t worth that for you, I get it. But I personally just only buy games when I am absolutely sure I’ll be playing them. I’ve found that’s worked out quite well for me so far!
BTW, a solid 2D Mario experience with online play is Mario Maker 2. It’s got a campaign full of Nintendo-made levels, and it’s now possible to play with friends via the online. If you change your mind on selling your Switch, it’s something you should consider.
