Your statement about SSD availability and performance does not fit with my personal recollections of history. Smoking fast SSDs were completely vanilla items you could order from anyone in 2015. For example, the Intel DC P35/6/700 series. Each of these was as good as 25 HDDs for linear reads and more like 5000 HDDs for random reads. The U.2 specification for putting SSDs in boxes was formalized in 2011. Outside of the mass market, cloud builders had private SSD designs long before 2015.