That depends on the functionality requirements of noSQL customers. I posit that writing a new, indepedent RDBMS that implements the latest SQL spec with the same maganement and replication capabilities as the incumbents is on-par with trying to compete with WebKit and Gecko. The SQL Server engineering team at Microsoft is about 1,000 people according to my friends on campus.

Whereas implementing a Key-Value Store is orders of magnitude simpler. There is plenty of room for innovation, of course - but you don’t need 1,000 people to build-on some new replication scheme or distributed backend.

If you take a simple NoSQL system then tack on things like object scheme support, JSON blob value indexes, node graph support, etc you’ll end up with an RDBMS analogue.

So the barrier-to-entry to building a NoSQL system is much lower than a RDBMS - and for many companies they might decide to build their own rather than extend an existing system - leading to more completion but not necessarily a better product.