I like the idea of something being backwards capable with DS/3DS but I'm not sure this is it. If they somehow made it work I'd definitely buy it though. Ultimately this is framed around carts that people already own though, and I think they'd be better off with getting the catalogue archived and available digitally on the 3DS.
There's a real problem with much of (not all) of Nintendo's portable DS/3DS software in that playing them legitimately on any future device, even where they could be released digitally, runs into the 2-screen thing. At least with the Wii motion inputs can be mapped to buttons, but DS/3DS software is largely mapped to specific hardware. You could have an accessory frame to hold Switch vertically and divide the single screen in two, I guess (which is pretty much what the 2DS did in terms of screen usage), but so much of 30 years of their portable back catalogue is locked away (outside emulation). I'd love to see them come up with a DS classic or whatever in a few years that gets as much of that catalogue available digitally as possible, and also the GB and GBA. At least the 3DS catalogue (and a chunk of the GB) is available digitally.