Re: So, if an enterprise buys ten 200GB SSDs it could then choose not to buy fifty 200GB HDDs.
Because they are shooting for IOPS and not maximal space. Even fast hard drives may only get 500 IOPS. A single SSD can easily provide 50,000 IOPS.
But beyond your question, everything in enterprise storage is about reducing storage utilization. Most SANs offer deduplication and compression. Coupled with an SSD tier, many businesses realize they need less total storage with newer technologies yet still have very high performance.