Methods like smart delivery + cloud saves, (and backwards compatibility) work just as well across a normal generational gap, as Xbox One and Xbox Series X demonstrate. The user's software, save data, NNID, etc carries over seamlessly to the next gen device with no need at all for smartphone-like soft successors.
Yes, but the game must be developed independently for both if the gap is too big, I thought I clarified that to you when I quoted the japanese dev, and you also know that apps can only be supported up to a certain version of smartphones unless they are upgraded. As the gap gets greater, so does the ability of software to stay compatible on all machines.
Anyway, here is the quote I was looking for: https://gamrconnect.vgchartz.com/post.php?id=6022102
Last slide here: https://www.nintendo.co.jp/ir/en/events/130131/05.html