As someone who just picked up SICP to see what the big deal was, I think that the choice of Scheme actually does add something: the ridiculously minimal syntax adds a kind of
clarity, making it clear that things you thought were
fundamental aspects of programming are really just language features. There is
so little syntax to learn that virtually everything you learn is a lesson in program
structure - because there's barely a "language" at all.
I was already familiar with Python when I started SICP, and the simple syntax caused a kind of fundamental shift in perspective. I had absolutely no idea how the Python parser worked. I couldn't model it in my head. I just knew that there were all these special things - methods, loops, iterators - and I could try and reason about how they'd work together, but I didn't really have a mental model of what was going on.
Scheme, in contrast, was so simple that they tell you more-or-less how the parser works in the very first chapter. I solved many of the first chapter's exercises on paper. I couldn't - and still can't - imagine running a Python program with paper and pencil. For the very first time in my life, I actually had a mental model of what exactly my code was doing.
And the super-minimal syntax made so many things I'd understood as totally distinct atomic concepts could actually be treated as related. The shift from f(x) to (f x) was a transposition of a single character - but it actually changed the way I was looking at the concept. f(x) was static - you specified a particular f, and applied your choice of x. (f x) - and suddenly f and x are objects of similar status. Expand and evaluate the element on the left. Expand and evaluate the element(s) on the right. Plug the stuff on the right into the stuff on the left. They follow the same rules! You can have (f (+ x y)) sure, but you could have the f as a compound procedure too! Or on the question of loops and recursion - loops had always been something I understood as a particular special construct, totally unrelated to recursion. Then - oh. Of course. They're both just ways of calling the same procedure multiple times. They're both recursion, just with different structures.
Basically, I think Scheme is serving two purposes here. The first - everyone learns assumptions from their first programming language. That's why you should always learn a second one, ideally a quite different one, just so you can see how there can be entirely different ways of thinking about and structuring the same concepts. So a language that's nobody uses is actually the perfect language to teach how to Structure and Interpret Computer Programs, because it helps you get past the pre-existing assumptions you'd unknowingly picked up about how to do that.
And secondly, because the syntax is so simple that it helps you really, clearly understand what your program is doing, what various ways of doing things mean. I hadn't even realized before picking up SICP that "learning to program" was a distinct skill from "learning how to code." They were the same thing - you learn the programming language, and then you just use the tools it gives you. Not knowing how to write something to solve a problem was mostly just an issue of not knowing the features of the language, or the available libraries, well enough.
In the first chapter, I was introduced to a simple subset of an already simple language. It was just parentheses, definitions, and conditionals, and a set of expansion and substitution rules basic enough I could work them out on paper. I didn't even have loops or lists. And it became suddenly clear that while I'd taught myself to code from reading language tutorials and the excellent Python documentation, I had never actually learned how to program.
So what I'm saying is, as an amateur programmer who was familiar with Python and not much else, I'm extremely glad that they chose to write it in Scheme instead of Python.