Of course. I've worked in and around the OSS DBMS market for over a decade. But the numbers were really big. It seems to me that out of a 2015 US $30B DBMS market Oracle and MS were collecting something like US $29B. The rest of the market,which included products like MongoDB and Cassandra was close to a rounding error. (I don't have the Gartner report handy, sorry.)
It's popular on HN to focus on OSS products, but there's a very large proprietary RDBMS market measured both in terms of revenue as well as users. That's how Oracle and MS got to be the behemoths they are today.