The problem isn't that simple use case. Although even in that case, they already had '=' as an assignment operator, and they could've easily kept it like the majority of other languages do instead of introducing an inconsistency.

The more major problem with the walrus operator is more complicated expressions they made legal with it. Like, could you explain to me why making these legal was a good thing?

  def foo()
      return ...
  def bar():
      yield ...
  while foo() or (w := bar()) < 10:
      # w is in-scope here, but possibly nonexistent!
      # Even in C++ it would at least *exist*!
      print(w)

  # The variable is still in-scope here, and still *nonexistent*
  # Ditto as above, but even worse outside the loop
  print(w := w + 1)

If they just wanted your use case, they could've made only expressions of the form 'if var := val' legal, and maybe the same with 'while', not full-blown assignments in arbitrary expressions, which they had (very wisely) prohibited for decades for the sake of readability. And they would've scoped the variable to the 'if', not made it accessible after the conditional. But nope, they went ahead and just did what '=' does in any language, and to add insult to injury, they didn't even keep the existing syntax when it has exactly the same meaning. And it's not like they even added += and -= and all those along with it (or +:= and -:= because apparently that's their taste) to make it more useful in that direction, if they really felt in-expression assignments were useful, so it's not like you get those benefits either.