Is my assumption right that the reliability of these programs is based on the proficiency and expertise of your father, rather than being a platform/language issue? Or do these platforms come with complexity that makes them more fragile in your opinion?
So yeah, it's more about the newer platforms having more hardware and software abstraction layers, so more things that can break and they have much higher power consumption.
In our applications, the weakest links are always the more "modern" communications modules, of which we've used Bluetooth and Cellular/IoT. In both cases, they are high-level SoC devices, with all kinds of capabilities and the ability to run BASIC or Python scripts on them. But they turn out to be much less reliable than our assembly code on the basic micros we use, and are the source of almost all device faults.
To illustrate the real-world advantage of the way we do it: our biggest competitor (a significant player that has shipped thousands of devices globally), uses a more modern/high-level platform (I don't know what exactly - it may be Arduino). One of our customers was given one to try out. Like most of our customers, they wanted data uploaded to the web app every 60 or even 30 minutes in order to have precise visibility of their soil moisture throughout the day. Ours can do that, running on just two 3.7V Li-ion cells that last for 1-2 years per charge. For the competitor product, to upload more than twice a day, it needs a solar panel, which needs to be mounted high up on a post above the crop. But that's a deal-breaker for this grower and many others, due to the use of pivot sprinklers and machinery.
So you are down to C knowledge, a datasheet and a register and pin map, which many people in their 20s definitely knew how to handle 10 years ago, when I used to work for Freescale/NXP. To expedite that further you can go with Processor Expert, which will generate C code and abstract much of the datasheet swordsmanship. Can be finicky to setup, but the "drivers" you get (C routines) benefit from long debugging sessions and lessons we learned about the "proper" way to use a peripheral, sometimes using info from the hardware teams that was not in any datasheet.
The thing to win with newer, power hungry platforms is the so called "race to idle". They are so powerful and have such efficient low power modes that the best battery life is obtained by cramming as much work in a wakeup as possible and then going to sleep, as opposed to having an exceptionally low power machine that is awake most of the time. So someone with an 80s paradigm might need to adjust to this reality.
Thousands of units is nothing and definitely not enough to (profitably) stay on hand tuned assembly.