Tim- have been using losts of my Dukes for going on 15 and even more years- and have never replaced a spring.
I do take care of the traps- they only get used 6 weeks a year, and are cleaned of crud and mud before being put away.
Never had a spring problem yet.
Never used to worry about the chains, but 2 years ago, had 3 chains break in 2 days- how odd! and once I relaized what was happening, I changed the rest of my chains to #3 machine. These could well have been on my older traps.
For me, biggest key by far, by far- is packing the inside of the jaws so hard, the dirt does not compress. Then, you get the trap to fire when the coyote has just about a full paw on the pan- resulting in a trap full of paw.