You lose SSD transfer speed, while gain _some_ hardware response latency.
NVMe response latency (on hardware side, not OS) is around 14-15micro-seconds on reads, and tops out at 200micro-seconds on writes [1]. Which is nearly 2-3 orders of magnitude below spinning rust. While Optane can halve this to 7micro-seconds on reads. The price/pref vs established NVMe tech is non-existent.
When I can spend $X/GB to get a 2000% performance bump, its hard to justify spending $X*4/GB to get 2050% performance bump. Most applications/data centers have yet to transfer to NVMe. Offering marginal gains a sizable markup is a risky strategy.
Furthermore Optane is nearly 2 orders of magnitude lower than RAM in latency, and 5 orders of magnitude lower transfer. So the idea of "non-volatile RAM" has yet to arrive.
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Also "Transparent to the OS" should strike fear in your heart. The OS's job is to manage hardware, memory, and caches. Why drop this?