I see where you're coming from, but that's assuming that PCs and NPCs are built with the same rules again- which they are not. And we touch on the "levels aren't a real thing in the world... but would I know that that gladiator is more powerful than that veteran? What separates them? What about my paladin as opposed to that knight?" Well there are some niche character features that can inform a PC of the target's AC, HP, CR, etc... but I'd prefer to make that some knowledge that can be attained by encountering the NPC in question, whether by fight, by demonstration, or by discussion ("can you tell he's a dangerous opponent by the way he holds himself?").