If I understand this right, with Intel's Optane you will eventually need to write everything to HDD because data collection happens at steady pace and the cache size is limited.
So as I see it you'd be constrained by 32GB Optane modules today, but they will eventually (one, maybe 2 years) be 2 TB modules like the Samsung 960 Pro modules are today. And an M.2 port is really just a PCIe slot so you're looking at systems with maybe 32 TB of Optane storage on the high end within the next 5 years.