Bought My 87' MTD 22" mower used in 2005 to moiw my room mate's yard when I moved to WA. Last year I did a complete rebuild of the self-propelled drive and cables, it looks and runs like a 4 year old mower now. When I bought it everything was original, including the blade. I've used it about 4 times this season to keep up a friend's yard, it still runs great.
I think a lot of it is the environment. Most people I know their yard is not horrible, but the back yard I grew up in ATE lawn mowers for breakfast! The thing was a giant erosion field/sink hole built on top of an indian spring! We had hidden dangers everywhere - snakes, stumps, tree roots, rocks....mud pits would randomy form in the yard and the grass would STILL grow over them. It was beautiful looking (especially considering I was "mudding" Self-Propelled walk-behinds to get it done), but heaven forbid you run a lawn mower over it. I've broken crankshafts, torn up blades, gummed up SP drive mechanisms in the mud.
In that yard...
First we had an 84' Lawn-Boy that was probably the best thing for that yard! THe only problem is it had drive cables and a stop switch that would constantly fail and prevent the machine from starting. However, it still stood as the only mower that could eat a well hidden stump with just a drop of throttle and some shrapnel. We only ever replaced the blade twice, it even ate a piece Rebar in another yard once (yes, REBAR) and still chugged away happily. It's still out there now in someone elese yard, probably eating their dead bush stumps, green with glee.
On the other hand, we had a Southland that shook it's flimsy-self apart, another Southland 20" that lasted 15 years with less service than the Lawn-Boy, and a Snapper that always got something else wrong with it after finishing the yard. Funny, the bad Southland once tried to bury itself in the muddy portion of the yard, whereas the Lawn-Boy would just coast across, throwing mud everywhere from underneath. Good thing I enjioyed fixing those beasts, I spent a lot of time "toughening" those mosnters up, but there was always something else to go wrong next.
So I'd put about 15-20 years on the average lifespan of a piece of equiptment presuming the owner fixes it rather than tosses it just because the pull cord broke or a wheel busted off.