Java introduced generics in version 5 which was a game changer, but what has it done since then? Is there anything in 6 - 8 that isn't just syntactic sugar?

Generics are also just syntactic sugar because it helps you to build type safe collections. But it would also work without it.
I think 6 and 7 had some major tweaks under their hood. Type inference, try-with-resources etc.
Maybe streams and lambdas will be seen as a game changer in some time like generics seem today.

If you think everything in 8 is just syntactic sugar I'm not sure there's much hope for you...