But as the article points out, capacity per dollar/watt/cm^3 is increasing exponentially for all of these storage mediums, and is expected to do so for many years to come. However, throughput and latency per dollar/watt/cm^3 is not improving nearly as quickly.
This means that throughput and latency will be the scarcest resource in the future, and regardless of the demands of your application, a time will eventually be reached where SSD has enough capacity at $X, but HDD does not have good enough throughput or latency at $X, so you switch to SSD. And eventually, the same logic will apply for SSD -> RAM.
Well HDDs never really increased in access time in the last 20 years [as they state] because the problem is basically rotational latency, so get a better speeds than a 15k RPM disk you need a 30K RPM, 60K RPM, 120K RPM disks etc which would be crazy.
SSDs are pretty new and are very fast despite several problems that the makers haven't quite worked out yet. Random access seek time can be improved by adding more chips. Also seek times will no doubt increase with faster clock speeds and reduced feature sizes in the same way CPUs and memory do now.
SSDs are lower power than HDDs and Memory is high on power usage not to mention the savings by having fewer servers.
SSDs are simply too new for people to design for them, if you look at what http://www.rethinkdb.com/ are doing, or the TRIM feature, or log based file systems, there is quite a lot to be done to update the software people use to take advantage of SSDs. It has all been written with the limitations of HDDs in mind.