100k mile life? Really? My 99 NBS 4.3 Vortec has 255k. I KNOW it is original. By buddy has another '01 4.3 with close to 400k miles!
I also have 3 other GM trucks with 240k (GMT 400,) 208k (GMT 800,) and 168k (GMT 800) miles. I don't know the history on those as I bought them not that long ago with all those miles, but when I've looked under there, the catalytic converters look factory. If any one of them fails, I'm getting a factory one from a parted out truck!
My brother has a 90 C1500 and my Dad has had a 89 C1500, and I don't recall any of them failing catalytic converters. Both over 150,000 miles each.
The only catalytic converter I know personally that has failed was a nephew's S10 with 4.3 Vortec that got plugged. He's not too good about maintenance. I told him he needed to figure out why it had plugged, but he just cut them off and kept trucking, LOL.
I got a little over 100K out of the factory one on this truck, but the ones on my Suburban are still kicking at 24 years old and 160K+ miles. My dad's '06 3500 has over 250K on the original cats. I think I'm just particularly unlucky with cats (among other things). The Magnaflow doesn't seem to be terribly clogged up, but it's rattling around pretty good in the housing.
I did consider just deleting the cat entirely and having Black Bear alter my tune to disable the rear O2 sensors, but I think I've decided against that. I don't want to smell like gas every time I drive the truck, or worry about passing state inspections. Plus I don't think anyone makes an "off-road" catless Y-pipe for the '96+ V6 exhaust system so I'd have to have one welded up if I went that route.