I know you have answered this question some years ago, but I think about the databases that have recently "triumphed" like MySQL, MongoDB or SQLite and they are free databases. Also in the Java world we have the example of Hibernate (ORM) and Apache Hadoop (framework for Big Data).
In my opinion, if you want to grow quickly and become a must-have in Java development you should release your code. I am not talking about what would be better for me but probably for your company and especially for the ObjectDB database. I mean, whenever I decide which database to choose for my projects I will consider pros and cons of ObjectDB and other databases and choose paying or not. But if you want ObjectDB to be a consolidated technology in the Java world you should release the code. You could even have a Community Version and a Commercial Enterprise Version (with more features and support).
Have you thought about it? Is releasing ObjectDB code in your future plans? Will ObjectDB always be commercial?