You're thinking of how Virtual Console games worked. You could transfer over Wii VC games and still play them in Wii mode, but you could only get the native Wii U VC versions of games you already had on the Wii at a discount instead of just getting them for free.
But other than that, digital Wii and DSi games could be transferred without a hitch.
Oh, I see. It's been a while so the details were foggy.
True, but I don't know if I'd call it spotty if there's a physical limitation preventing it. Because I have nothing better to do, let's look at the ratios for major systems, not counting first generation (NES, GB, PS1, PSP) :
- Nintendo has 10 "sequel" systems. 6/10 are BC (5/9 if you don't count GBC as a new generation). If you ignore systems that have completely different storage mediums, that's 6/8 (or 5/7).
- Sony has 5 "sequel" systems. 4/5 are BC, though only early PS3 models can play PS2 (but props for getting Vita to play PSP games anyway despite the lack of a UMD drive).
So if - and only if - you count different storage mediums as being a legitimate reason for not having BC, they're about on par, with Sony being slightly ahead.
The only case of physical limitations would be N64 to GC, and even then if they really wanted to they could get it to play cartridges too, like with the Game Boy Player.
SNES & N64 could've done it, especially the SNES since even if we excuse the lack of backwards compatibility due to time restraints or something (not very believable, considering Famicom released in '83, SF in '90, I think the better part of a decade is long enough to squeeze that feature in; at least Genesis released a converter for SMS), the system had a revision unlike N64, so they could've included it then.
Wii U was especially egregious because I think only hacked ones can play GameCube games.
As for Switch, it may have an excuse not to play physical versions of past games (at least portably, they probably could've done something with the dock if they really wanted to) but there's no good reason not to carry over digital games. I think even at least some PS4 and/or PS5 games like Legend of Dragoon (technically an enhanced version of the game including Trophies) are honored if you had them on PSP/Vita/PS3 (I hate to praise modern Sony).
There's no good reason Super Mario Bros should be locked behind a subscription service on Switch when you bought it on Wii, then paid to carry it over to Wii U (not even counting if you had it for 3DS as well).
I'd say Nintendo is definitely spotty with this, but I do think they want to preserve as much goodwill moving forward as possible, so at a minimum I think we'll see digital backwards compatibility, but probably both.
