I think it's not really too early to discuss this. I've seen this before, actually. For a long time in project X it was said that discussions were too early, and then finally a decisions was needed and then it was made relatively quickly without much further discussion, leaving quite a bunch of opinions from people out of that decision.
For one I think the way we look at versions may become different. Before a version in Java was a major thing and surrounded by a lot of fear. Libraries didn't dare to support newer versions of Java, and companies didn't dare to upgrade, with as a result getting further and further behind over time. Who doesn't know that one company where battles have been fought about upgrading from e.g. Java 6 to Java 7.
But look at browsers now. Do we see much fear around upgrading from say Chrome 62 to 63?
If Java releases new major versions every 6 months, and who knows, maybe even more continuous than that in the future, then maybe the version fear will disappear as well, and most people would always be on the latest version or somewhere around the latest version.
If that were to happen (it remains to be seen, of course), then the target of any library could become the version of Java that's current at around the time of release of said library. For instance, for JAX-RS 3.0 that could be say Java 13 or 14.
Just my 2 cents