"In a data center people would generally prefer to have big storage capacity as they can access drives in parallel for speed. The greater density allows more storage for the power and cooling required."
It's perfectly true that you can get more IOPs (and throughput) by installing more drives at the cost of power consumption, cost and data centre footprint. However, there is one think hard drives can never do, and that is consistently achieve random read latency times of less than about 6ms. With SSDs you can get down to tens of microseconds (or, realistically, on a SAN hundres of milliseconds). If you have an application which is I/O bound on random reads, then only SSDs will get you out of it once you;'ve exhausted the practicalities of caching. (Random writes are not such an issue - enterprise arrays will cache those in NV store).
As for 4TB drives in the enterprise, then we have had real issues with the reliability of high density drives. They are OK for some semi-archival uses (for which you'd never use SSD anyway - at least not for the forseeable future). However, use them hard, and they fail at a much higher rate than the lower density enterprise drives. Then there is the problem that re-building RAID sets with such high density drives takes a very long time as capacity inevitably outgrows throughput (read/write throughput goes up in proportion to linear bit density, capacity as to square of linear bit density). That pushes you to double parity which removes some of the capacity advantages and can also impact performance.
SSDs will erode the top end of the enterprise drive market as prices drop. Realistically, write endurance is not going to be too much of an issue. Enterprise drives fail too, and once they get to the limit of their practical operating life, failure rates start to increase and swap-outs happen more frequently. Of course it's generally covered by maintenance contracts, and exactly the same thing will happen with enterprise SSDs.
Disks won't go away, at least for the forseeable future, it will just get pushed more and more to the bulk storage area.