True, there will always be optimised forms of hardware. However, those will be for geeks like us who care about such stuff.
There were always be faster, albeit more expensive, technologies which we will utilise as cache on faster buses.
IIRC some mobile devices now treat their solid state memory flexibly between OS/cache/storage...?
I guess at some point, memory will be so fast that faster memory for certain applications won't be necessary in practical terms.