Tool for the job
Spinning rust is not dead, and might not be for years to come. The important part is to understand what goes where.
Broadcast cache for a production company? We add an 8T disk every week, would be quite costly to flashy that and completely emptied of any meaning - it simply doesn't matter if the material can be read at 20 or 200 times the running speed, it's needed at 1x.
Your MP3 collection? My experience tells me, it's much more likely the medium will die without warning if it is flash, when you plug it in after a few weeks of poweroff.
I have yet to see flash in grown-up sizes, that could give disk a run for the money on very sequential workloads. Could easily be, that Disk is still around, when flash is reduced to a footnote in history.