It's about speed yes - but it's also about capacity, coolness and quietness. Of course it will be expensive but I can really see this coming into it's own in small builds, notebooks and the like.
I have 5TB in my desktop and it weighs a ton, requires drive cages and looks Ugly AF. SSD's can be mounted anywhere. 1TB was close to $1k AUD iirc when it came out and now it's half that. 2TB are $1200 I imagine these will be the same and reduce as well when the 4TB are released.
Yes NAS is an option for a lot of people. Clouds are great if you have quality infrastructure (we don't) If you're on the move all the time this could be more suitable.