We already had this, it's called Intel Optane Persistent Memory and Intel killed it off last year: https://www.intel.com/content/www/us/en/products/docs/memory-storage/optane-persistent-memory/overview.html
The memory speed was slightly slower than DDR4 but the benefits didn't seem to outweigh the downsides. I think it probably kicked a lot of ass for specific use cases (eg. in-memory database that needs persistence), but the market was too small. Plus, SSDs are getting so ridiculously fast that it would put pressure on a product like this too.