is mixing actually open-ended enough for that to even mean something? Like, is there always a way to do it better, or is it more like, at some point you figure out how to do it right, and then the world's most skilled composer couldn't improve it?
I don't think I could differentiate mixing levels above Essence, anyway.
The world's most skilled composer is probably not the world's most skilled mixing engineer, but putting that aside, in theory there should be a point at which a mix is so good that nobody can improve it (beyond maybe changing it to appeal to some particular people's personal tastes more and less to some others', since it is a bit subjective as well). There are basically infinite things you could do and infinite degrees in which to do each of them, but at some point, you're definitely in the right ballpark and any changes you can do are so minor that nobody can objectively assess whether they're improving or worsening the mix, and eventually they're so minor nobody can even hear what the differences are. In practice, I doubt a mix even close to that good has ever been made.
This mix, for example, is way better than Essence and likely one of the best mixes ever in this approximate genre, but even this mix is doing some things with minor detrimental side effects to achieve the overall sound with the appropriate aggression, fullness, cohesiveness and clarity for the song and I suspect it would be theoretically possible to get pretty much there in other ways that have smaller detrimental side effects. I do think it's the right decision to have them if that's what it takes to get this huge, powerful sound though it's always a question of how you manage the compromise, and I always have to make a compromise that results in a less impressive sound while the detriments are more audible than this because I'm not quite this good, and also because I'm mixing recordings that weren't this good to begin with.
That being said, there is a point at which the mix basically serves its purpose in conveying the musical intent of the song, and I guess that's when most people stop caring about the mix. I'd say Serenity fell only slightly short of that, and everything I've mixed after Serenity has more or less gotten there.