Techman-001, you are 100% correct, except for one thing. You talk about learning from your past.
Yes, if you are educated and proficient with STM devices and dev tools and supply chain, I'm sure there is almost no reason to use any other microcontroller in a volume product. Today. The cost of these devices can be significantly cheaper, even, than other devices that are way inferior in most ways (other than maybe some electrical specifications in some cases).
But if you learn from your past, you know this is a temporary situation. In X years from now, STM32 is obsolete, too! Perhaps not even due to technological advancement. It might be some other whim of industry or business or some other butterfly effect.
Assuming (some) people will still want a simple way to interact with hardware in ways that don't need the latest cutting edge speeds and memory and floating point math, the Arduino platform may avoid this fate. Either the AVR from 1970 will not become obsolete, because Arduino community will continue to use it in enough volume that Microchip/Atmel will not phase it out. Or the community will adapt and incorporate better more modern microcontrollers to the Arduino platform in a way that the average user will not have to learn much to use the new greatest and latest Arduino Pikachu. (One day, maybe Arduino Techman is created, even... with an STM32 on the board?)
Arduino means you don't have to keep learning in order to do the same things you always did... and not get bent over when that part becomes obsolete and starts to cost 4-5x as much. You can use that hammer for a whole bunch of nails. And you can learn to use the latest greatest fancy biscuit jointer if and when you have to. After you do so, you might like it so much you make everything with it.. but one day it is also badly obsolete and overpriced unobtanium... When that happen, you might still have a trusty hammer, lying around, which is at least good enough to drive nails. And in the future, the latest Arduino has more speed and capabilities and less bugs than it has today, even. The Arduino platform is sustained and evolved by its user base. It is not chained to a particular micro. I kinda doubt it is ever going to deviate from AVR family, but who knows.
If you are a high school or university, do you get to take choose your classes to learn STM32 or modern AVR or modern PIC? Not usually. You would need to find the teachers that continually learn and teach the latest cuttest edge devices and completely change and update curriculum. They can offer classes using Arduino, because it's simpler and because you can bet it will still be relevant in 10-20 years.