The author of the study makes a lot of arguments based on factors that are easily changed, like the configuration of an SSD. However there are a few basic technological trends:
1. Disks and NAND flash are both getting more dense at fairly comparable speeds - disk has been getting cheap faster than flash lately, but may have a hiccup in the next few years. Where flash has conclusively replaced disk is in applications like iPods and mobile where "enough" storage is cheaper than a single disk. (the iPod went flash when 2GB of flash reached $50, which is the price of a micro-disk) It's not going to replace disk for high volume data storage anytime soon.
2. With today's disks and chips, a hard disk drive has a relatively fixed cost (the cost of the factory amortized over the number of drives produced) and similarly flash has a relatively fixed cost (cost of fabrication plant over the number of chips produced in its useful lifespan). The number of bits on each doesn't really matter - that's why packing them more tightly makes the bits cheaper.
3. Disk bandwidth for 7200K drives isn't going to go over say 300MB/s anytime soon with today's perpendicular recording technology - if the disk is moving past the head at a constant speed, the only way to get more bits through per second is to pack them more closely on the platter. And the best you can do by spinning faster is a factor of 2, at 15K. (and those are very low capacity and very expensive)
2 and 3 mean that flash can easily supply cheaper bandwidth than disk - it's the SSD maker's choice how widely they want to stripe data over the chips in the drive. (64 ways isn't unreasonable) There's a huge advantage today, and it will stay the same (see #2) if flash chips don't get faster, and get bigger if they do. (at some point getting that speed may require paying for more flash than you need, but at that point a single disk will be bigger than you need, too)
For years flash was getting slower and less reliable (requiring more complex error correcting codes) as it got denser - that's partly why it got cheap so much faster than e.g. RAM, where you can't cut those corners. The next generation of flash (3D NAND) may reverse that for a while; in addition SSDs are finally a noticeable fraction of the market so there's an incentive for vendors to make faster flash. (3 years ago SSDs were 3% of the flash market, and the rest went into iPods, phones, and removable drives and cards - SSD vendors had to make do with flash that was designed for systems where you don't care about performance)