Then there is literally no such thing as a "level-independent challenge" that can be described mechanically. Nothing like that exists. The only things you can possibly have are non-mechanical, and thus cannot be applied, not any more than what I've said above; they are pure concepts, like "stop a ritual" or "negotiate with the ruler, which can only achieve application by giving them mechanics. By your own standard, the conversation is pointless.
I prefer to have a more productive conversation, where we can speak of things that lack explicit application to a specific game, but which can still be fleshed out to a significant degree without any reference to target numbers, modifiers, or whatever else.