As opposed to most cprgs which would simply have all monsters of the same type being the exact same level. I fail to see how that's a negative.
Some (mostly bad) crpgs do that for sure, but I doubt that they are the majority and specially not a majority among best crpgs.
To make exploration, looting or fighting interesting and diverse, there are many options that TW3 developers for some reason decide to not implement: Adding new creatures and loot by level, or create truly high level zones filled with totally different high level minor enemies/loot, or maybe the most perfect but costly option: adding uniqueness in enemies and loot. There are many other hybrid options.
In the codex top 50-70 crpgs list, for example, most games (or all?) use some of the aforementioned options. And this independently of the game subgenre, later Ultimas, most blobbers, Infinity ones, Gothics, Daggerfall/Morrowind, etc.
The witcher 3 bestiary and enemy diversity is good, but they are always the same enemies (besides bosses) across the world and independently of the player level, and looting don't helps at all as you admit. It wouldn't be a good design even if the combat was on the Dark Souls level, looking the scale of the gameworld and quantity of content. And I think I understand one of the reasons for what developers made the game in that way... The general design and focus of the game was not that of a true open world crpg neither a character or party progression type rpg, but a heavily story and fixed character driven experience. All was build around Gerald and the stories. And from gameplay perspective the result of this design in minor enemies fighting, exploring and loot search it's a poor decision that affect negatively the experience. It's true that a Geralt that wasn't a superman/witcher capable to kill easily near every creature from the start, couldn't do much sense in lore perspective, but looking to game-play, it's a poor option.
An extreme example with Morrowind, very complex, far superior to TW3 model in this context, to mention a game with similar free roaming, a big world full of minor quests (far more than TW3...) in which combat is far from perfect and looting is not balanced at all, but you can see however many options used to make the exploration and minor fighting far more diverse, challenging and rewarding: Adding truly different high level zones, implementing a complex leveling system adding new, higher level creatures progressively but without remove completely the low level ones, including a huge amount of unique enemies (all 3000 npcs, many creatures or animals) and so many unique items, placing many loot in high places or way down underwater so the use of non combat skills and tools to reach them is mandatory, etc.
Might and Magics or Wyzardrys use a simpler enemy scaling and different creatures by zone model. Gothics use the most clear zones-by-level model in any game that I can remember, etc. Few games in the codex top 70 do it worse in these contexts than The Witcher 3 did, which loot and enemy scaling/placing is simply mediocre or bad. And that's even more striking looking to the attention put by CDPR in writing, script... and graphics.