NIC - I had a '78 F-150 w/300-6 also....but I traded it in TWENTY years ago on an '87. I replaced the matched Pair of fan belts on it too, with belts from My Ford Store, I'm using the old ones on My home-made lawn vacuum I run about every weekend now for several hours. So yes, belts CAN and frequently do last a lot longer than expected. But I've also had cheap autoparts store belts fail in less than a week. I too buy only good parts now. I DO change My oil more frequently than every 10,000 miles however, especially on a vehicle I use for frequent short trips, especially in cold weather.
Don't know why Your Ranting....I'm just stating fact about the ISO program We implemented at a former employer. We're also discussing the same problem where I work now. We use a LOT of o-rings and rubber deteriorates in the presence of Ozone at a pretty fixed rate...rubber also doesn't like sunshine. You can add things to the rubber to slow the deterioration down, but it never stops. Heater & radiator hoses the same thing. Even vinyl electrical wire insulation. At the same company I helped achieve ISO I also Inherited a year's supply of laser-cut vinyl LOGO decals for Our machines that were self-adhesive and already almost 4 yrs old. They had a 1-year shelf life. Yes, They WOULD NOT STICK. But the 28 yr old "IHC" decals I got from FARMALL back in '79 which also had a 1-year shelf life WILL stick, but they're the soak-in-water and slide off the paper backing style, not self-adhesive vinyl.
Also, There's a LOT of difference between what a Person is able to get by with at His own risk and what a company will risk which can and will harm their reputation. Selling shoddy old service parts is Not a good plan. I'm sure You expect decent parts from Your Cub Cadet dealer don't You? So the person buying belts @ MTD better not buy a 10 yr supply!