The inner plugs should be good for 100,000 km. The problem is rusting into place, as is evident in the removed items. Even if these are long life plugs, they probably should be removed and cleaned every service - I just replace mine, they don't cost too much in the scheme of things - 4 x plugs, air filter, oil, filter plus gearbox and Cardan oil changes are the regular cost items - not expensive if you do it yourself. I buy the engine oil in 20 litre drums so it aggregates to really good prices over a few oil changes.
A smear of anti-seize on the threads on re-assembly for the inner plugs ensures they come out easy the next time.
Make sure you get the plug caps on the inner plugs, properly - they are a right royal bitch to get back into if they are not on right.