SSDs haven't replaced spinning HDs yet. And probably won't for slow read but massive storage. They handle the functionality of general use with 300GB HDs but that's some ways off current multi-terabyte spinning disks. And those might increase storage capacity faster than SSDs. So server OSs will have to maintain compatibility.
And NVM tech will probably be more limiting in storage to current RAM than SSDs are to spinning HDs. To replace even 30% of RAM/Storage in 5 years seems like a stretch.
HDs haven't replaced tapes yet. And probably won't for slow read but massive storage. They handle the functionality of general use with 20GB tapes but that's some ways off current multi-terabyte tapes. And those might increase storage capacity faster than HDs. So server OSs will have to maintain compatibility.
