My replacement's training cost is well into the millions, and they wouldn't give me that as an FRI; so it's a saving! The training pipeline's often kinked, and even if it's working fine, you don't need an LCR pilot who can't do anything; you need to be able to keep your high level multi-qualified QWI/QFI guys with experience and lots of platforms under their belts.
I'm not sure there are any branches or trades where a new guy straight from training can immediately have the same effect as a guy with 20 years of experience, but in the aircrew world, that gulf is more pronounced. It's really difficult to put a price on the knowledge and experience of a 20 year FJ pilot, but considering it's going to cost you millions to get a replacement to the point where he can safely land a Typhoon or Lightning II, never mind operate it, paying him 6 figures to stay around isn't a bad option.