What you are talking about is RAM tiering which is done in large enterprise multi-CPU superclusters. For a consumer machine, it doesn't make much sense. To get the best out of it, the client software would need to support such architectures explicitly, and customer-grade software can't even get basic memory management right. Its also very expensive and not space-efficient. Current SSDs are more then fast enough as a backing store.
At some point, when the persistent RAM is fast enough, we'd go away with the current tiering (RAM vs disk) and just have a single storage device. Its how operating systems have worked for a while now anyway. But it will still take some time