The key to understanding their longevity lies in the fact that they were the earliest high-level languages invented at a time when all software was built for serious long-lived stuff viz. Banking, Insurance, Finance, Simulations, Numerical Analysis, Embedded etc. Computing was strictly Science/Mathematics/Business and so a lot of very smart domain experts and programmers built systems to last from the ground up.
Don't be so optimistic.
One of these days I was at a small grocery store in my hometown, and saw the screen of a POS system I had built in the late 90s with Visual C++ 6 running on Windows 10. The guy told me he got the system from the previous store owner he bought the shop from, it did most of he wanted it to do, and that his son is the guy who do things like upgrading the hardware, and writing small python programs that talk to the same SQLServer database to extend the system and implement some integration, so he sees no reason to get another system.
Nice. I hope anything I've done lasts that long, but most of my jobs have been solving long-solved problems by yet another player in the market, so I expect it will all get decommission sooner or later as these companies either die or change hands.
The computers themselves were also so expensive that most businesses did not buy them, they leased them.
> a time when all software was built for serious long-lived stuff
I sometimes lament that most of the code I've written for work will probably be retired before me.