The spinning disk era is coming to a close, and I welcome it! The issue is that while storage capacity has, for decades, increased almost exponentially, the actual performance of the HDD has remained virtually flat. A typical HDD spinning at about 7200 RPM can store 4 TB of data or more, but can only serve about 150 seek operations per second. Physics, she is a bitch, you know? So while you might be able to store 500 million files, it takes a month to copy them.
Everywhere I look, Enterprise or "performant" storage has moved to SSDs. We moved our DB servers 4 (5?) years ago to SSDs and saw at least a 95% reduction in query times involving disk operations. (EG: not cached) For us, spinning rust is the new tape; SSDs (or RAM) anywhere performance matters, and spinning rust for archival use. For our session cache, we use a RAM drive.