Does it blow anyone else's mind that database servers are so powerful nowadays that you can just load half a billion rows into a SINGLE TABLE and pretty much expect things to work? Obviously there are caveats but all in all a nice chunk of RAM backed by a large SSD goes a long way.
As a long time skeptic of the NoSQL craze that seems to have (happily!) peaked, I have to say that no, I'm not very surprised by this.
Databases (real databases) are using indexing technology that's been proven over not just years but decades of use and refinement. log2(500000000) is still only 28. Double that and it's only 29. And these databases' indexes are typically btrees with much higher branching factors than 2.
SSDs live long enough. You get magnitudes more IOPS performance out of a single SSD than a RAID of 15k disks, it consumes magnitudes less power, it costs less, and it needs less maintenance. It's a complete no-brainer these days to put an SSD in your database server.
