It's an admirable goal, and we'll see whether it works out in practice. Remember, this entire thread is about JDK updates. Java also explicitly has forward and backward compatibility as a design goal, but then sun.misc.Unsafe turned out to be a bigger headache than most people thought, and upgrading to Java 11 will require many companies to update core dependencies, and unfortunately a lot of companies would rather pay for Java 8 support than put in the maintenance effort.

Even if there's 99% compatibility, if upgrading requires anything more than updating the compiler, even if it's just adding a runtime flag and the upgrade effort should be minimal, you're going to see significant resistance from companies with large codebases.