You must not have many rocks for the guards to last that long. Good life on the sickles too. You probably found it cost effective to buy new bars complete with new knives and bushings.
Life of a disc mower depends on a few things, including dumb luck. If the cutter bar is a gear bed that run in oil, It will last a very long time if the oil is changed once a season and you are lucky and careful in finding leaks from bad seals. Cutterbar needs skid plates replaced when they get worn, or repaired by welding fresh steel on them. Gear box needs oil changed also.
Gearboxes fail when a seal fails and the oil leaks out. Some makes have some sealed bearings in them which may fail with obvious results.
The Vermeer-Lely and AFIK the New Idea-MF-Hesston mowers have individual cutterhead gearboxes in the cutterbar and are driven by a shaft. On the Vermeer-Lelys, these have sealed bearings and bevel gears that need to be repacked in grease seasonally. Someone on her suggested that the grease is similar to cornhead grease, don't know. I'm guessing that the NI-MF_Hesston have the same setup, but not sure. These types of cutterbars can be rebuilt more or less indefinately at less cost that rebuilding a gearbed cutterbar. Sometimes it's cheaper to buy a new cutterbar assembly than to rebuild, IF the rest of the machine is worth the effort.
I found it cheaper to buy a new gearbox than to rebuild for a Vicon once. For all the above reasons, used disc mowers aren't worth much unless their history is known and they are creampuffs.
New Hollands have individual cutterhead gearboxes which run in oil and are shaft driven with little short shafts which telescope into each other. The NH gearboxes are bolted together, using a great deal of torque, to make a cutterbar. The NH is heavier than most other makes.