I wouldn't expect migrating from 17 to 25 to be an awful lot of work. The hard bump was moving from Java 8 to 9+ (11, typically), due to the introduction of the module system, removal of APIs previously shipped with the JDK (e.g. JAXB), etc. Things get much easier once you've passed this hurdle. Adopting 17 posed another, usually smaller, challenge due to reflection not working OOTB like before, but I'm not aware of other, equally disruptive changes after 17.

You haven't done it, then. Just the re-testing of everything in these large companies is terrifying and complex. Particularly the real-time stuff like Kafka. Most companies refuse to update unless absolutely necessary as they see the solution as an investment that has to produce. Security concerns have changed that attitude somewhat, but you would be surprised how much re-work is often necessary. The frustration of sorting it all out is not worth the money, imho--I'd rather just do something else, so I am.

Except he didn’t say “for ten years” but “in ten years” referring to the extreme length of time companies take to upgrade