Flash has a place, but don't retire rotating rust just yet...
Exactly. However fast flash may be, and however suitable it is as storage, to bring more flash capacity online (at current densities), you first have to build a multi-billion dollar fab - and that fab has capacity limits that keep the cost of flash up, no matter what you do to try to bring it down.
Rotating rust (and yes, I know it hasn't been rust for a long time, but I used to be in the drive industry and that phase has fond memories for me), on the other hand, only takes a couple hundred million to incrementally add another line to an existing factory, and incrementally add capacity (and adding a new factory isn't that much more expensive). So the drive manufacturers can ramp up capacity much more incrementally than can the flash folk, and thus keep $/GB costs lower.
Add to this that data-at-rest (think Google, facebook, twitter) is the fastest growing storage category, and you see why hard drive manufacturers sell all the drives they make, and will continue to do so for many years to come. "Cloud" (you call it what you want, it's data that people want to keep but not necessarily access very often, aka write-once, read seldom) is where the market is. For these guys, capacity and density at the lowest possible cost are what matter, and there are new drive technologies coming online that will increase both incrementally (at little or no cost increase) over the next 10 years (helium filled, SMR HAMR, 3D, and so on). A 10GB single spindle drive isn't around the corner, but it is probably only 5 years away (never mind how long it'll take to format or fill one of these monsters). Flash won't be able to scale down in cost and up in capacity fast enough to deal with this market in the next 5 years (it may, ultimately, but that isn't the current trend).
For hot data (AKA enterprise data, on-line, etc.), flash is emphatically taking over (if it hasn't already done so, at least in the Request-For-Proposal space...). In fact, at least one drive manufacturer is no longer designing enterprise-type (2.5in, 10K+ RPM) drives any more, because flash is clearly going to own that market in the not-too-distant future. Building a 2.5 inch 10K+ RPM drive is lots harder (thus more expensive) than building a high-capacity 3.5 7200-or-less RPM drive, so from the drive manufacturer standpoint, 3.5 capacity drives are where to focus the effort.
From that enterprise storage (online storage) viewpoint, the article is mostly correct - but that market is NOT where the vast growth in the storage is these days. It's where a lot of the cash is going, but not where the majority of the bytes are going.