I disagree. Even if the concepts of spell slots or levels are not explicitly "in-game" concepts, I would argue that spellcasters are aware of the differences between kinds of spells and their relative power level.
With respect, you don't get to disagree with the facts of how my games generally run. Might as well disagree with someone saying wood comes from trees.
You may, if you wish, argue that it should be otherwise. You can argue with me as if I hadn't already considered the basic positions one might take of this... decades ago, already. I don't think it'd be productive.