It's quite impractical for them to "support" SSD. The way (I assume) their infrastructure works would basically mean that the speedup you get from SSD would be nullified by the rest of the IO needs imposed by their stack. That is, if they were to simply move one service (in this case, EBS) to SSD.

The only way I see this happening is if they start adding servers with SSD, where all services running on those servers support SSD too. This would basically mean a separate cluster, fully based on SSD. I don't see this happening soon.

They don't have to move to support EBS with SSD and mix and match. They could have RDS instances with SSD only option and EBS based on SSD. They can charge significant amount for those options, some might even pay for it.