Yup, it's not about reskinning or hi-res in the first place. It kinda starts from inside out, beginning with the core gameplay, new attacks for monsters, new moves for players, and things like properly textured/slightly bigger maps to fit the modern stuff in.
Nothing keeps people from using hi-res textures and models with RMQ. There are, however, other people who do these eyecandy things better than we could do them. With the exception of sounds - they are tied very closely to the gameplay extensions we did, and 3rd party "hi-res sound replacements" are ultra rare.
You could probably use Rygel's or Starbuck's textures, and Error's or someone else's models with this pack. We used modified textures, more skins, and a few new weapons though, so these hi-res packs won't cover everything.
Yet theoretically, they could - the qc source is available, and you can very easily extract the textures a map uses, and see the skins a model uses, so you can precisely see where hi-res coverage is spotty or would need to be extended.
You are correct that eyecandy is not the core of Remake Quake... but you could still dress it up like you want, and play it in Darkplaces with bloom if you like that
Or you could put all pink flats on the walls if you are wired that way, and run it in ezquake, and it would probably still look better and not feel as cramped as the old maps - to me, having played them for a year or so, dm6 and e1m2 feel a little like a kid's toy, or Mini Legoland, in comparison to their remake versions. Don't get me wrong - the toy version has a certain charm. But you could never put things like the new grapple into vanilla DM6. It's too small, folks.
Many benefits of re-mapping aren't going to be immediately apparent. The main one is a more solid structure, a more modern build style strictly on a larger grid. Players won't necessarily notice that - but it is priceless for anyone who wants to texture, re-texture or even modify them. The map sources are going to be open, so anyone can mutilate them at will
RMQ is basically a re-imagining / re-building / strengthening of the raw materials, or the game's skeleton. Which skin you put on it is up to you.
Edit: It's not really about "killing Quake", either. More like "Quake is great, but can we knock the dust off and teach monsters some new stuff, and get some crazy moves on, and everything 30% bigger? Yes, I think we can!"
