- Open JDK has not historically been a production-ready deployment. It was a lower-performance, "reference" implementation, explicitly NOT recommended for production deployments. This changed around JDK 9, but see the next point.
- in spite of Java's attempts at backwards compatibility, migrating non-trivial projects to new major releases of the JDK has always been painful and slow. Many projects are still on (or recently reached) Java 8. For example, Spring Boot released support for Java 9 just 6 months ago.
- many Java applications are large enterprise applications (many dependencies), and enterprises want stability and long-term support of infrastructure components. It's likely that not all dependencies will get migrated to a new JDK within 6 months, even less that the application will be migrated, and that says nothing about rolling it out to production.
- and production is when you really want security updates, etc. And Oracle's plan is that the last scheduled security update would be a couple of months before the next OpenJDK release, so you really only have ~4 months of a supported, updated JDK release.
- there have been various security problems with Java, even with Oracle's support (particularly client-side, which is becoming less relevant)
So Oracle is moving from a slow-moving cost-free model to "hop on a rapid upgrade treadmill on a release we've historically told you not to use".
So the simple answer seems to be "pay oracle for support".
- Then you get into Oracle's [abusive? domineering?] licensing practices, like requiring licenses for every CPU in a VMware cluster.
- Some clients have been told by oracle reps to try enabling feature X, then brought up for license violations because feature X is commercial-only.
- Working with Oracle is problematic enough that clients don't even want to enter into a conversation with them about the consequences of the JDK license change for fear of getting on Oracle's radar.
The community appears to be stepping up, but there's still a lot of concern about whether they will be able to provide production-ready, secure builds, across the many platforms Java supports. (Notably, Solaris support appears to be lacking...)