Since the only advantage hard drives have over SSDs is cost, and flash gets about 30% less expensive every year, the only way for hard drives to remain relevant is to also get less expensive over time. Bigger hard drives have always been the key to low storage costs but bigger hard drives lead to slower disk arrays.
As disk drives get bigger, the number of drives needed to build a storage system of any given size decreases and fewer drives simply deliver less performance. The table below shows the number of disk drives, and the aggregate performance those drives can provide, in a 1 PB array based on 225 MB/s of sustained bandwidth and 150 IOPS per drive.