Buying an SSD for your notebook or desktop is nice. You get more consistent performance. Applications launch extremely fast... ...However, at the end of the day, it’s a luxury item. It’s like saying that buying a Ferrari will help you accelerate quicker. That may be true, but it’s not necessary.
Whether a Ferrari is considered a luxury or not also depends on the user/usage...
However, the point the article seems to miss about replacing a bunch of 15K dirves with a single SSD is that the primary reason for multi-drive RAID is typically capacity as well as redundancy. You don't replace a RAID array of 1TB+ drives with a single SSD of ANY kind.
While mechanical storage requires redundancy in case of a failed disk, SSDs don’t. As long as you’ve properly matched your controller, NAND and ultimately your capacity to your workload, an SSD should fail predictably.