You use it only for the one class you are allocating a million times per second and let the other ones be GC, often this technique is used within a single heavy function with a tight loop

Most of your application probably doesn't need it and the tight loop might not be big enough to be worth using JNI to write in a non-GC language (as well as maintaining another compiler stack for)

Ideally the runtime should allow you to manually manage memory when you need to, like some keyword that lets you manually keep track of an object lifetime. But as far as I know most GC languages don't have that