I suppose it depends a lot on why you write or what you want to get out of it. A large part of my motivation is getting better. It’s satisfying to me to feel like I’ve improved in some aspect or another.
So I find good writers inspiring. Why wouldn’t I compare myself to them? I do the same thing in everything else, too. If someone is better I know I can be better, too. Aiming at some dark unknown that might or might not be possible seems much tougher to me
Sure. The perfect novel is of course good at everything, not just prose.
But - and that’s back to my definition of good writer - good prose is good prose no matter what else is going on. You can write about grass growing and paint drying and it’d still be a fascinating demonstration of your superior command of language. If you actually were that good.
So, and of course that’s still just my opinion, but for me you can’t ever write a good book if you can’t write good prose. But you can easily write a good book if your characters, plot and so on are sort of “lackluster mediocre” (not a perfect book though ^^).
That’s why I say the core ability for a good writer for me is writing good prose. The rest - I don’t want to sound too dismissive about it; they aren’t easy things to be good at either, but it’s not just as important to me.