The facts is, data density on solid state has already surpassed data density on magnetic/spinning media. We already have 4TB SSDs at SanDisk, and 8TB and 16TB drives are on the horizon. I highly doubt you'll see HDD do 16TB in a 2.5" package anytime soon. Also, when you factor in power requirements, the economics start to look very favorable to SSD, not 10K or 15K RPM drives which are used in latency-sensitive applications. And, in another unexpected and non-intuitive twist, SSDs have a much higher MTBF then HDDs, around 2.5M hours vs. 1.1M hours. Wear-out has become a non-issue, even in the harshest environments. There are SSDs that are rated for anything from less than 1 Drive Write Per Day (DWPD) on up to 45 DWPD during the length of warranty (usually between 3 and 5 years).
Bottom line, SSDs are a far better economical choice in many cases. Of course, this only matters where economics count. Enthusiasts are already seeing the benefit in lower prices and higher densities, but the OP is not about them. Economics is not about "can the consumer afford it" as much as it is about "does this enable me to lower my TCO in the data center". We are there already.