An album by the two of them would have to be a certified classic, otherwise they would lose face. They're probably aware of this on some level and even could have gone as far as experimenting in a studio.

But they also know that the expectations for an album by the two of them are going to be unmanageable. If the album is simply 'good' it's going to be looked at as a miststep at best or a sign that they've fallen off. They both likely don't want that even though the appetite for the album is large by their fans.

Also, they connected 27 years ago on illmatic and had some great moments around that time. That is a generation ago. They clearly had chemistry then but they're both artists with different demands, different needs, different egos.

As disappointing as it is to accept, the moment has simply passed and the Nas / Premier album is now on the same level of missed opportunity as the Heltah Skelter and Detox albums.

It should have happened but for realistic (and disappointing) reasons it likely won't.