Where Optane could've been used: distributed storage. The whole game there is about how fast can you ack the writes while writing as far away as you can. The company I worked for in this specific area had used pmem when that was available for that specific reason.

I'm no expert on costs of these things and especially wouldn't dare to predict how these costs could've behaved in the future, but my guess would be not that the technology had no use, rather that it was too expensive for what it was good at, and the second best was cheap enough to warrant choosing it over Optane.