If one tries to extrapolate the "price per storage unit" for both technologies, it might happen that both will run into a physical limit before the price will catch up. The harddrives are already there (superparamagnitic limit) but so far manage to trick their way around it. Lithography has (optimistically) only one more order of magnitude to go.
Actually HDDs haven't hit a limit, they figured out how to octuple capacity to 24TB for 3.5" and 8TB for 2.5"s
I think SSDs will fill a different role than hard drives, and the reason will be reliability vs. speed.
I think that SSDs will trump the fast hard drives like the Raptor, but I think in reliability people will still want slower terabyte drives to dump all their media on. It's just more convenient than dumping it on DVDs. Though that might change if the recordable media significantly improves a generation(s).