Nikon made a trade-off (as they first did on the D300S years ago) to accommodate two different formats to attract upgraders. I am sure they realized two different formats is a compromise solution, but to introduce a camera which invalidates ones entire memory collection would also have been a downside to many and a barrier to sales.
For those of us who have used it, it is clear that XQD is superior to CF (and CF-Fast) in every way. It is a great new technology, I hope to see XQD in the D400 also and then every pro/semi-pro cam from then on in. XQD is able to work with your buffer to provide a service that is the equivalent of a bufferless camera, which is a great feature.
It is also pretty clear that CF is dead-ended, so we have to move somewhere.