If that were so simple. What if your software gets deployed on an application server? If you upgrade, you need new support contract, this costs money plus leads to possible deployment/config changes, for instance going from JBoss 5 EAP to JBoss 6 EAP is not a pleasant experience as almost all configuration was radically changed.
What if part of your application is audited and certified by some regulatory body, when you upgrade, you need to go through the process again.
Java value is stability, you can assume your software will run for 20 years without touching it.
No one is forcing anyone to upgrade. What worked on Java 8 3 years ago, will work on Java 8 in 17 years. If your team have extra requirements (audit, certification etc.), it seems it sells enterprise software. All those things cost money, but I assume you also charge money. So, what's exactly the problem? It is a bit naive to expect to get ultra stability for free from your suppliers, but charge top dollar from your customers...
By only providing fixes and security patches for a few months -- after that you need to move to the next version (with all that entails testing and adapting wise for large codebases), or you pay for enterprise support and get support for years.
Before you could go to any version and expect it to be supported for years.
Now, if you dare go to some version above 8, you're supposed to either pay (for long term support), jump directly to the next version after a few months, or stay without security fixes.
What if part of your application is audited and certified by some regulatory body, when you upgrade, you need to go through the process again.
Java value is stability, you can assume your software will run for 20 years without touching it.