Well, RPN with parentheses, as a layer of syntax, would be easy to deal with. That's what Lisp backwards would be.

  ((a b +) (a b) defun)

  (((list pop) print) list while)

Yeah, the Japanese incorrectly put verbs at the end of the sentence! Just like Yoda does. Asian he must be. Incorrect they all are.

But seriously, yeah computers are naturally 'Stack Machines' [almost] always, where they need the arguments pushed onto a stack before the 'verb' is executed.

Even if we don't think about stacks, but just the raw data dependency: before we can perform an operation, we need to determine its operands.

So if we translate that sequence to language, stating the operands before the operation does follow the non-negotiable evaluation data flow.

That's why we have interpreters and compilers: So we can make code easier to read. FORTRAN was originally invented as a way to let humans think in terms of parenthetical groupings for math and functions, specifically to _avoid_ reasoning in an RPN way.

What programming systems used RPN prior to 1957?

Doing ASM ~ "RPN reasoning".