If "full stack" means electronics up to JS, then there are probably quite a few people who can work at all those levels. Although a minority, at least they can understand a "fuller" stack than most, unfortunately.

I disagree, it takes lots of time but it is possible.

Personal example: I have an electronics engineering degree that was 1 semester short of a physics degree, so I learned quantum mechanics, electromagnetic field theory, transistors, and how to create a CPU (I even created a CPU out of simple gates and way too much wire wrapping). I love computer software, so I learned assembly, how to write compilers and operating systems, and libraries. I have configured network hardware and written network software at various levels. I've also used CSS and implemented UI/UX. I've written code in many programming languages, including JavaScript, Python, C, C++, Java, Ruby, Rust, Common Lisp, and Scheme. I eventually got a Computer Science degree as well.

None of these things are magic, and the info is relatively easy to get. You simply have to keep learning and be willing to try new things. It can be fun, too.

Yes, today it can be helpful to specialize at any particular time in your life. But I think it's best to use that as a launching pad to branch out.

Jack of all trades, master of none. I don't think it's bad at all: at least you can specialise at something (quicker) whenever the need arises.

It's relatively easy to learn a new language (for example) when you think, "this is like x, this is like Y, etc."

I guess my key point is that you always need to keep learning, and don't box yourself in too much. Ideas from elsewhere will show up... being aware of them makes it easier to use them and be ready for them.