Note the context in which I said that comment: "these big HDDs". I wasn't talking about HDDs in general. I was talking about the sorts of leading edge, big drives being discussed in the summary. They aren't ending up in typical consumer machines as boot drives. They are the sorts of drives that are being purchased as an add-on to an existing system. Moreover, some of these drives, such as the WD Reds, are being specifically targeted at the NAS market. So, at least in the context in which my comment was intended, I stand by it.
But even if we discuss the industry in general, I'll admit that my thinking is pretty much how you painted it, since I do think that you're overestimating how long the majority of PCs will ship with HDDs as their primary drive (no doubt, they'll linger on for many, many years, much like the floppy drive). Thinking ahead a bit, until the next shift in media formats occurs (i.e. until optical media dies and typical media distribution moves to downloading/streaming), we're probably pretty safe in assuming that most consumers will continue to only need around the same 150-500GB they need today, though many will obviously buy 1TB. That's where it's been for quite awhile, and it's been pretty stable for quite some time.
Before I go any further, let's just look at some actual numbers. I popped over to PCPartPicker's listings, grabbed the first 10ish drives when I sorted them by price per GB, and have provided the range of prices below to give us both a representative sampling of what sorts of prices are realistic right now for cheap drives:
Best Overall: $0.03/GB for 3TB vs. $0.31/GB for 240GB
1TBish GB capacity: $0.05-$0.06 vs. $0.36-$0.47
500ish GB capacity: $0.08-$0.10 vs. $0.31-$0.40
250ish GB capacity: $0.12-$0.20 vs. $0.31-$0.40
150ish GB capacity: $0.15-$0.25 vs. $0.41-$0.50
What we can see from these numbers is that a typical consumer buying a typical HDD with a typical capacity can typically expect to pay 2-5x more per GB than someone buying a high-end drive with a massive capacity, whereas the variance in cost per GB across SSDs is much smaller and has uniformly been dropping at a steady pace. I.e. While a price advantage does still exist for HDDs, that advantage is smallest in the segment of the market where the everyday consumer is located and is rapidly shrinking. That fact has allowed SSDs to position themselves as one of the simplest, cheapest, and most significant performance upgrades a consumer can choose when buying or upgrading a computer (even my non-techie father insisted on an SSD as his primary drive when I was helping him configure his last computer), and as the gap continues to close between the two, HDDs will soon be relegated to nothing more than bulk storage.
Already, computer manufacturers are starting to drop HDDs from their product lines (e.g. Apple's entire product line, sans the Mac Mini, has already switched), and the HDD manufacturers clearly see the writing on the wall, which is why they're starting to market specifically towards the prosumer market with products like the WD Red.
I do think that we're still a few years away from the tipping point, but I'd peg it at around 3-5 years out, rather than the "many, many years" you suggest. Though, as I said, I expect HDDs to be with us for many, many years.