Honestly, I think this is now a good idea. I haven’t necessarily felt this way in the past, but it makes sense. It would seem to be a very easy thing for them to do in terms of the Steinberg license key itself. I think the problem would likely be how their internal support and management would have to change to accommodate this. My understanding is that the products are managed by different teams/divisions, have their own staff, etc… and Steinberg appears to have a corporate structure where there are more defined boundaries between teams/divisions. So there may be a business/structural and even accounting situation they’d have to resolve.
But as a general concept, I like the idea that a current Nuendo license runs the current Cubase. Or, as others have suggested, merge the product lines into one product line and Nuendo is essentially “Cubase Ultimate”… although I’d suggest keeping the Nuendo name. I like it. 