It's not hard because the changes are small and gradual, and you have to update your JDK every quarter anyway -- whether you use the current version or an old one -- just to get the security patches. It's just a matter of how that work is spread. It's likely that updating once every 3 years is more work overall. Plus, you've done that already back when there were still major releases, and the semi-annual feature releases didn't get a new integer number (8u20, 8u40 etc. were big feature releases).
Plus, keep in mind that LTS is a paid service. What appears to be "free LTS" isn't really LTS, but builds of the OpenJDK Updates projects. Those builds serve as the basis for various paid LTS offerings, but in themselves they don't actually maintain the full JDK. OpenJDK Updates just backports stuff from the mainline version. If you're using a component that's been removed from the mainline -- and you probably do or else you'd upgrade -- then that component is not maintained in OpenJDK Updates (so-called "free LTS") because there's nothing to backport from.
The only version that's 100% maintained for free is the current JDK (15 as of today).
Plus, keep in mind that LTS is a paid service. What appears to be "free LTS" isn't really LTS, but builds of the OpenJDK Updates projects. Those builds serve as the basis for various paid LTS offerings, but in themselves they don't actually maintain the full JDK. OpenJDK Updates just backports stuff from the mainline version. If you're using a component that's been removed from the mainline -- and you probably do or else you'd upgrade -- then that component is not maintained in OpenJDK Updates (so-called "free LTS") because there's nothing to backport from.
The only version that's 100% maintained for free is the current JDK (15 as of today).