I grew up with Arduinos, never used 555 because it draws too much current for what it is doing. I get how it once was a popular thing, but if I need a simple delay circuit or simple logic that needs to e precise I do it discretev if ir needs to be more complex there is any number of MCUs.
I was 100% self thought and teach electronics in art university now. And I have to say I can't really confirm your suspicions about "the kids", sure many stay at the module level (totally okay, they study arts not electronics), but many don't. I had a student who over the course of 2 years built a brain wave reading circuit with a specialized instrumentation amplifier IC, to filter out grid EMF she built an opamp based notch filter and that woman had nearly no help from me and no prior education in the field. That analog stuff isn't going away anytime soon.
It is a bit unfair though as one is comparing new MCU to ancient parts. For example the TLV9301 is a updated version on the 741 and is superior in basically every possible spec, but people still use the 741 out of habit. And if you need a lower power discrete timer, the 555 is not the best way to do it in 2024. There are a huge number of options.
For art projects I totally get using a MCU. You're probably only making one and the product is the art. The engineering just gets in the way so minimizing man hours, which includes the time to learn to do the thing, is critical. It will be tough to beat a MCU on that metric.
That is what I meant: the 555 is a habitual option by people who grew up with it. The arduino can be an habitual option by people who grew up with that.
Not every project is mass produced or must be highly optimized when it comes to size, cost or power consumption. People use what they know, and what they know depends on when they grew into it.
Arduino is very much less about the board itself, and more of a software framework. Every single "Arduino compatible" board beyond the standard Uno or Micro have a bunch of macros essentially just telling the compiler what the standard Arduino pin definitions go where. That even extends to boards like the 2560, since I know the port registers and the silk screening on that board are completely different.
I was 100% self thought and teach electronics in art university now. And I have to say I can't really confirm your suspicions about "the kids", sure many stay at the module level (totally okay, they study arts not electronics), but many don't. I had a student who over the course of 2 years built a brain wave reading circuit with a specialized instrumentation amplifier IC, to filter out grid EMF she built an opamp based notch filter and that woman had nearly no help from me and no prior education in the field. That analog stuff isn't going away anytime soon.