Actually i do not see a future market for 7200rpm and above disks in the 3.5" format. That will die out.
Either it will be fast small 1.8" and 2.5" disks (10k rpm 2.5" disks like velociraptor), or it will be fast slow disks aimed at capacity, like the WD Green series. 5400rpm sounds like the ideal tradeoff. Even with a lower spindle speed; high transfer speed is possible. High enough for any data files that require streaming (movies, music) and fast enough to transfer alot of files. The sequential speeds of the WD Green are pretty good, and i do not think harddrive manufacturers will invest alot of money in a dying sector.
SSDs will take over sometime, not that HDDs will disappear. But they wouldn't be used as often. As they will loose the position of system disk they will only be used as second "data" disk, with all the installed software and operating system on the SSD instead. Many people would not need an HDD anymore.
Then there's a problem, the current price of HDDs is only maintainable because of the high production volume, so producing an HDD is efficient. But that can change if the demand for HDDs drops significantly. Then the HDD makers will have to increase prices, or change their strategy. I'm convinced this should prove to be a positive development for consumers. I'm just hoping it would go faster.
Current strategy seems to outgrow the SSDs by capacity. Making them look really small. But howmuch do you actually need as a system disk, seperating your data files? Probably alot less than HDD makers are implying with their current capacities.