ANTs Software’s primary focus isn’t really even on DBMS any more. Even so, it just announced a deal to replace Informix in a large retail chain’s in-store systems. (In its 1990s heyday, Informix wound up running in-store systems at an impressive list of major retailers. Of course, Informix was long ago acquired by IBM.)
EnterpriseDB has probably passed ANTs in the DBMS plug-compability business. And taken together they’re still pretty small. Even so, plug-compatible DBMS replacement has to be taken seriously as a (possibly) emerging trend. Economically, it makes all the sense in the world.