My mom was a school bus driver for 30+ years. She saw the evolution from slow, stubborn, overstressed, short lived gas engines and manual transmissions to early noisy, slow, but indestructible diesel engines and manual transmissions to powerful, wonderful to drive diesels still with manual transmissions to the power-robbing automatic transmissions they bolted to the diesels, to the "always something wrong" diesels towards the end. It got so bad that they couldn't retire some of their older buses because the newer ones were just so unreliable. And my mom was literally the very last driver that drove the manuals, so for a long time, she got stuck driving those because those were the backups. And those didn't break. Ever. the IH DT466 with the 5sp was the best, but many were the DT444 (the same basic 7.3L they sold Ford for a time).
Since she drove for the Special Education dept., they had wheelchair lift equipped units that were more expensive to replace, so they had to keep them in service longer. I'll bet some of those old ones are still in daily use as public transit... many go to South and Central America and are reborn. I cannot imagine they'd want any of the newer ones, though.
This was our '92 4700:
Old MODOT truck, had been through three owners afterwards, had over 650k miles on it, the cab had been rusted out and welded back together multiple times, but that DT466 and its 7sp manual OD box were still in perfect working order. That truck could sit for 6 months, outside, and it would fire right up without hesitation on the very first cylinder that landed its power stroke. Didn't smoke, didn't hardly use any oil, and boy howdy did it have some IMMENSE pulling power. We worked that thing harder than a drunk girl on prom night, and it never skipped a beat. Sold it, still running today.