It may be an odd question, but I'm wondering for how long will programs made in Jabaco work.
As long as the Java VM will support Java Bytecode. There is no end in sight.
Seems Jabaco is not being updated much anymore,
http://www.jabaco.org/board/p2921-new-version.html
but I'm wondering if there could eventually be incompatibilities with new versions of Java that well nobody would be around to fix?
The base for your development is the underlying framework and this part is very open for everybody. Sure, the compiler is important, but only for the language design and the performance and not for the compatibility. The interface to the framework should be compatible to VB6 and the upcoming improvements.