My view is that HDD's die when SSD's get the production cost lower than an HDD. While companies will probably exit the HDD market, this just props up the others, until only one is left. When that happens, that manufacturer will probably just ditch all of the R&D (no point in future development after a certain point) and just churn out cheap drives on their existing equipment.
Taking tape drives as a point of reference, i'd expect that HDD's will be with us for the forseeable future, certainly the next 10 years, probably the next 20 although likely dropping into niche markets like SAN's, which have a good use for low cost high volume storage. Of course, if the cost of making an SSD suddenly drops by half (which I can't see happenning) then all bets are off and the SSD probably dies within 6 months.