The WiiU wasn't grouped to the Wii because the Wii cannot play any WiiU games.
The Xbox1 featuring forwards compatibility which allowed it's users to play severely downgraded versions of a Xbox2 game would still have the same software library, I mean... these machines are getting closer to PC all the times, it can't be too far off to simply have PCesque settings to turn down resolution and kill some of the effects, I mean you look at something like Overwatch on the PC, the Minimum specs to play that includes a GTX460 while there is people who would claim their eyes would bleed on anything less than a 1080TI that doesn't take from the fact that a game which can potentially run at 4k/300FPS (300fps is the limit I've hit at 1080p, not sure why) but yeah... the same game can be experienced on a PC with a Graphics card which can be bought for £15 https://uk.webuy.com/product-detail/?id=sgranvigtx4602&categoryName=pci-express-graphics-cards&superCatName=computing&title=nvidia-geforce-fermi-gtx-460-768mb
For comparison sake, that's a game which runs on a card which is 1/8th the power of what some would consider part of the PC master race, close to a decade old card, which launched 5 years before Overwatch, still capable of playing the game.
I just mean... the Xbox 2 will not be 8x the power of the X1 so the older machine should be able to provide a similar experience to what a GTX460 user would enjoy with Overwatch compared to someone on a 1080TI
It's extremely unlikely though that the base Xbox One is going to be forwards compatible with the next Xbox, it is already struggling to run current gen multiplats like Assassin's Creed Odyssey and Just Cause 4. For that kind of cross-compatibility would require that Xbox Two would be severely held back. Graphics may scale, but CPU side less so, especially given how slow and weak the Jag in the Xbone was... in 2013.