In many trades, salary is a very small fraction of the training bill. Training a Fighter Pilot has to be around 2M$ in flight hours alone.
Your logic behind the replacement of a Senior NCO is a bit flawed. It is true for thhat specific position, but you also need to train and replace everyone down the chain that gets a promotion message as a result of the release. And the experience level of the person is not te same.
Let's take the example of a 2500 Hornet hours Major that releases. This guy had 2 combat tours, a fighter weapons instructor, instrument check pilot, multiple tours in an Operationnal Squadron. Yup, to fill the void, all you have to do is promote a Capt to Maj. But that Capt probably doesn't have all the qualifications and experience the Maj had. And you'll have to train another fighter pilot that year at a cost of more than 2M$. Would a 400K bonus keep him for a couple more year? I think so, lots of the guys get out to go instruct in the ME where they make 200K tax free, so money IS a motivator in most cases.
Now, apply this to a large scale. Your training organization doesn't have to be as big, you can keep your experienced pilots in thre Op Squadrons instead of training units and your experience level increases (instead of decreasing like it is now).
Bottom line? Reduce the bullshit people have to deal with and don't shortchange your experience. Experience is worth a lot of money and takes a long time to build.