I have cleaned a many a plug over the years with the wire wheel on my bench grinder. Then filed the electrode and regapped. Learned to do that from my dad. I am only 35 and it was not until this year I actually witnessed my first true spark plug failure. It was on my daughters 150cc go-kart. It would fire fine cranking the engine over, checking for spark. The engine would run fine at idle and you could feather the gas and finally get some RPM, but it would not handle any load what so ever. Everything pointed towards the carb. I read a bunch of forums based on those engines and go-karts and that seemed like the go to thing. Ordered a new carb, same thing. Checked fuel flow, everything I could think of. Finally got a new plug because it was one of those special order metric jobs, and Vroooooommmmmm! I felt like a complete idiot!!! I had my spark tester on it and everything. It's just when it got under compression it didn't have enough fire to light more gas than at idle.
Moral of the story is a good cleaning and gapping and most plugs will last a long time.