I think the thing to remember here is that the reason we have SSDs at all on the desktop is due to the rise — and now dominance — of mobile computing. NAND flash solved the capacity-volume-mass-performance equation for mobile devices. Desktop computers do not need to solve the same equation, as mass (and volume, to a great extent) are entirely irrelevant.
Thus SSDs are — originally and primarily — a technology solution for the mobile space. Even though desktop users have embraced the technology, manufacturers haven’t forgotten which side of their bread is buttered. SSD R&D is heavily influenced by the needs of mobile users, not the needs of desktop users.
Compared to the average desktop user profile, the mobile user profile is even more heavily biased towards content consumption than it is for content creation. As a result, storage capacity isn’t nearly as much of an issue for mobile consumers as it is for desktop creators.
Further, many manufacturers (e.g. Apple) have fully embraced planned obsolescence and, as a result, pressure users to throw their devices away every 3-5 years and buy a new one. There’s no point in having vast amounts of storage in disposable devices.
Just like the growth of consoles retarded development of gaming, and explains why the dominant resolution is still 1920x1080, the growth of mobile has (and will continue to) retard the development of large capacity storage.
Looking forward, a big thing for mobile would be the unification of memory and storage — which paves the way for crash-proof, instant-on/always-on devices. Examples of such technology already exist, and the pursuit of PCIe speed/bandwidth is part of this. If that ends up being The Next Big Thing™ in mobile, then advances in capacity for SSDs may be slow for decades to come.
The gaming/enthusiast market is a rounding error that practically no-one cares about, so our desires/opinions do not matter. We just live off the scraps that are left on the table after the big-money markets have had their fill.
If vastly greater storage capacity is what one desires, then one has to look at the market segment that has deep pockets and need for such storage. That would be enterprise, not mobile. High-capacity SSD storage for desktop users will happen as and when it makes sense in the enterprise space. I suspect that the enterprise equation features capacity-power-reliability-performance?
Someone who pays more attention to the space than I do will have a clearer idea of the needs of enterprise, and will probably know what sort of storage tech is in the pipeline. If you want capacity, that’s where I think you should be looking.
tl;dr: SSDs are primarily a solution for the mobile space where capacity isn’t an issue. Look to other markets (like enterprise) to drive the availability of large capacity storage.