Re: What about 3D?
The price per GB of SSD is already down to around 5:1 mark if you consider like for like (1TB 2.5" 7200rpm disk vs. 1TB 2.5" SSD).
Flash also tends to be much more reliable than spinning rust. Sure, there's the write endurance limit on flash, but this is a complete non-issue in just about every sane use-case:
Consider that most of the tested SSDs survived 1PB (yes, that's _peta_) of writes, with some surviving as much as 2PB. To write that much data to them took between 1 and 2 years of continuous writes. To write that much data to a mechanical disk in random order, would take many, many times longer. Sufficiently longer that you would be similarly looking at about 100% failure rate over a similar write volume. Then consider that write:read ratio is typically less than 5%, and that a mechanical disk suffers wear regardless of whether an operation is a read or a write (bearings and actuators will only survive so many seeks and revolutions) you are looking at a real life expectancy of an SSD that only suffers wear on writes that will on average vastly outlive a mechanical drive.
Spinning rust is increasingly struggling to maintain it's relevance in most environments.