I just opened a Cubase 8 file on N 6.5 yesterday. Worked fine.
But what the OP is saying is not opening files, but that a Nuendo and NEK license should also be a Cubase license. And I could not agree more. It’s been suggested/asked for a number of times, unfruitfully.
I’m sure Steinberg has a picture in their mind regarding licensing and pricing for Nuendo and Cubase that makes sense to them. But I’ve never met anyone outside of SB that understands what that might be.