Optane was supposed to fill the gap but Intel never found a market for this.

Flash is still extremely slow compared to ram, including modern flash, especially in a world where ram is already very slow and your cpu already keeps waiting for it.

That being said, you should consider ram/flash/spinning to be all part of a storage hierarchy with different constants and tradeoffs ( volatile or not, big or small , fast or slow etc ), and knowing these tradeoffs will help you design simpler and better systems.