Did they not in the dark days of old buy enough to allow for attrition throughout service life?
Maybe, but I'm guessing (forgive me if I'm wrong) that the planned attrition rate was somewhat below 4 per year...
IIRC (and again, apologies if I'm mistaken), until the one lost in the Balkans, there had only been one Herc lost in recent memory (the one in the Highlands). In the last couple of years, there have been at least four.
Just goes to show how hard the Herc guys (and girls - in between press interviews ho ho) are working - great job you do despite the difficulties and dangers, long may it continue.