honest question; if the witcher 3 had either turn based combat (somehow) or had action based combat on the (mechanical) level of Dark Souls, would you regard it to be a 'proper' RPG? Is it's shitty combat alone enough to disqualify it as one?
You are going on a forced dichotomy there, Witcher 3 didn't need to be turn based and didn't need to be Dark Souls as an strategy game can be turn based and not be an RPG and a game may have strong combat and not be an RPG. Witcher 3 has alot of problems, the combat is pretty crappy and wouldn't hurt the game for it to be more interesting but this isn't even my biggest issue with the game.
Witcher 3 needed stronger support for mechanical storytelling and not just cinematic storytelling to be an interesting game from an RPG point of view. There is a complete split on the game of parts that don't interact well, the open world, the story and your actions on this story.
You're right. The Witcher 3 devs fucked up by going for a sandbox. It destroyed the gameplay, forced the decision to make the world static, to let you explore freely without consequence, the bland / repetitive flora and wildlife, the atrocious handholding, the terrible level scaling. That is, not counting the other faults that came with going for a larger audience, such as the shitty and easy combat, the gimped alchemy with potion usage in combat, the absence of need for preparation before most fights, no consequence for stealing... basically everything to prevent a new player from ragequitting. It is basically a really shitty Gothic clone in the gameplay department.
But, despite all that, it is still the best C&C AAA action-rpg since Bloodlines, with the best writing. (Fuck NV it didn't want to run for me.) It's also the best sandbox, because all quests have a minimum of interest to them, they're not fed-ex, but actually flesh out the world a bit. What you do in the world is also believable, taking contracts as a Witcher, while other games just put you in the position of the chosen farmer who will go from zero to god in a week, all while doing all the little menial tasks of random NPCs that ask for your help because they can feel your chosenness.
Most important of all, The Witcher 3 has characters that are realized in a way we haven't seen before in a AAA. Bloodlines, Alpha Protocol, KotOR, ... they have nice writing and shit, but the characters are brodudes and brodettes, they're funny and quirky and whatnot, but none of them show their humanity, their flaws, their doubts, their sorrows, in a believable way. The Witcher 3 did this, in a non-lore-dump way. Despite all its flaws, this has to be recognized.
And aren't the Codex's top games ones that are deeply flawed? PS:T, MotB had atrocious RTwP gameplay (and that fucking PST multiclassing, fucking hell). Obsidian games don't need a quest compass, there is usually one place to go, follow where the story takes you. KotOR is basically a fucking corridor. Bloodlines has 2 street cities. Fallout combat sounds cool on paper until you just aim for the eyes (it is satisfying though). Arcanum, heh, better not start (and I have one of its avatars, not for no reason). Divinity OS? Awesome combat, cringe worthy story and atrocious everything else.
Why are they liked so much? Because, despite their numerous flaws, they did something very well. You might not like that aspect of RPGs that they did well, and therefore hate the game, but that doesn't change the facts. We are some motherfuckers that will tough through some shit gameplay or story if there is some good experiences to be had.
