Because

1. Physical dongle tends to break, and when it does, they expect us to give them replacing parts

2. They do expect bug fixes-- especially calculation bug fixes-- as the bugs are discovered. It's hard to leave their production critical apps broken like that once you know that the bugs can cause monetary or even life loss.

Honestly, if they never need anything more from the developer, a perpetual license and never spending another dime seems fine. However, in modern times, OS vendors (especially one named after fruit) tend to break a ton of APIs and change rules with every "major release," meaning developers have to invest a ton of effort to at minimum meet all those new requirements every year (!) or else the app will at best look out of place, more likely look totally screwed up and exhibit sudden "bugs" due to the unexpected OS changes, or at worst, crash.

Then users are suddenly all over the developer to provide an update "so I can use this on Tahoe" or whatever, and unless the application is in its honeymoon period where new sales suffice to keep money flowing, the developer is gonna need recurring revenue in order to do recurring development.

> Why should users upgrade or keep paying you when they already bought what they need and don't need anything else?

Because things evolve and inevitably, hardware dies, and you can't get a replacement.

With an old "dumb" piece of machinery, when something breaks you can either repair the broken part itself (i.e. weld it back together, re-wind motor coils), make a new part from scratch, have a new part be made from scratch by a machining shop, or you adapt a new but not-fitting part. It can be a shitload of work, but theoretically, there is no limits.

With anything involving electronics - ranging from very simple circuitry to highly complex computer controls - the situation is much, much different. With stuff based on "common" technology, aka a good old x86 computer with RS232/DB25 interfaces, virtualization plus an I/O board can go a long way ensuring at least the hardware doesn't die, but if it's anything based on, say, Windows CE and an old Hitachi CPU? Good fucking luck - either you find a donor machine or you have to recreate it, and good luck doing that without spec sheets detailing what exactly needs to be done in which timings for a specific action in the machine. If you're in really bad luck, even the manufacturer doesn't have the records any more, or the manufacturer has long since gone out of business (e.g. during the dotcom era crash).

And for stuff that's purely software... well, eventually you will not find people experienced enough to troubleshoot and fix issues, or make sure the software runs after any sort of change.