Xbox One

Because we don't live in pre 2006. Nintendo has had full BC support for two generations of consoles now, and every generation of handhelds since the Game boy colour (being the second major handheld...), so there is no good reason that everyone can't offer it.

The fact you have to quote so far back in nintendos history just proves the point. Short of a complete change in input media (N64-> GC) or PSP-> Vita, there is absolutely no excuse.

Especially not for digital purchases. Holy cow, how did we end up in such a position that only nintendo feels its worth the consumers time to completely carry purchases over??

Change in input media just means you need another slot for the other input media. XBox One and XBox 360 run on completely different computer architectures, which means you need to build and support an XBox 360 emulator in x86. That's a lot of pineappleing work (and it may not be possible to get to a desirable performance level). Sony is only making it work by porting individual games over to x86.

Nintendo has done backwards compatibility in the past by emulating the previous generation on hardware that is fairly similar. The more complex stuff gets, the harder it is to do that. There's a reason why you can pretty flawlessly run every GBA or N64 game on PC, but you run into all sorts of problems with more recent consoles, and it's not that modern PCs don't have the specs to run those games.