It's interesting, my experience with companies here in the U.S. (mainly SMBs) are that they feel much more comfortable using a SaaS compared to adopting an internal NoSQL store. Granted most of these companies aren't highly technical, but I suspect for them using a SaaS is easy and doesn't require much internal expertise. Adopting a new NoSQL store though may require training, etc. Of course I concede that things may be different in larger enterprises and companies outside the U.S.

I would bet, however, that most of the clients that do business with Mongo or DataStax still intend on using/maintaining a relational system. I haven't really encountered many companies that's decided to completely dump their relational systems in favor of something else.