I don't really have a problem with continued 3DS support because it seems like a lot of these games wouldn't be an option for Switch. I don't think it's a 3DS vs. Switch decision for Nintendo, it's more likely 3DS vs. not doing them at all for most of these games. If Nintendo announced a Luigi's Mansion remake for Switch, people would expect a lot more work and effort to go into, and it might not be worth it for Nintendo at that point (especially if they've already got a Luigi's Mansion 3 in the works for Switch that will come out relatively soon, which could very well be the case).
I do question Nintendo's continued 3DS support in the sense that it doesn't seem like anything they're making can actually move systems, and with 3DS software sales slowing down, it doesn't seem likely that they'll be huge sellers on their own either. But as a fan, I have no problem continuing to play good games on 3DS as long as Nintendo and other publishers, like Atlus, continue to put them out.