Your entire post uses ignorance of physics as a bludgeon.

> The term "systems" seems a bit of over-reach, in line with your original premise.

No, it is precisely the right word. Which you’d know if you knew enough about physics to have this conversation.

> What is important, for the purposes of this discussion, is that condensed matter physics is a specific sub-domain of physics, rather than all of physics itself,

None of this makes sense. Condensed matter physics is an application of quantum mechanics used to model properties of macroscopic systems. I used it as an example of how macroscopic things absolutely do demonstrate quantum mechanical properties, and gave examples of macroscopic phenomena that we can only explain in terms of quantum mechanics. I never said that it is “all of physics,” but that’s a nice strawman.

> which exemplifies the fact that QM doesn't literally explain how or why classic physics ("Newtonian physics" was the iconic representative) arises from the principles of QM. In theory we "know" it must, but as it has not yet been convincingly demonstrated in general cases, insisting that conjecture is fact is problematic.

I even gave you a link to a general proof of the correspondence principle and described an approach for an alternative proof, and it’s telling that you didn’t even respond to that at all, and instead built a strawman to argue against instead. Neither of those are conjecture, they are proofs. Your ignorance of them doesn’t mean they don’t exist. You should stop arguing about things you don’t know. You may be the best philosopher in the universe, but you can’t do philosophy about topics you don’t understand, let alone those that you aren’t even aware of.

> I would dispute half of that. Scientific models are entirely about how, and necessarily unconcerned with why.

Semantics. Science is about constructing models that reflect reality insofar as we can measure it. A scientific model is good if it is consistent with existing data and accurately predicts the results of future experiments. For example, quantum mechanics doesn’t address how things happen, only what will happen. GR does the same, although in both cases we use words that ascribe some sense of “how,” but it’s more to help us talk about and understand the math, than intrinsic to the model. For example, we often say that mass curves spacetime, and that’s what gravity is, but it’s possible to reframe GR in terms that have nothing to do with spacetime curvature. e.g. it can equivalently be thought of as arising from torsion instead of curvature, or even as a field on a standard spacetime background; all of these are probably mathematically equivalent and no experiment can ever, even in principle, distinguish between them. This nearly identical to the issue of interpretations in QM, it just gets less attention for a variety of reasons.

> I would say that the notation arises from the distinction rather than the other way around.

I’m not talking about notation, I’m talking about the meaning behind the notation (which I suspect you probably don’t know). Either way, you’re missing the point. The point is that in circumstances when hbar is small compared to the relevant scale factors of a system, quantum mechanics turns into classical mechanics. In that sense, classical mechanics is embedded in quantum mechanics.

> But of course, and I may be mistaken about this and please enlighten me about your reasoning if I am, I presume you meant classic physics reduce to QM, rather than the other way around, which is what you actually typed. I think the idea that quantum behavior necessarily reduces to Newtonian mechanics is absurd, isn't it?

No, I meant what I said. In the macroscopic limit, quantum mechanics reduces to classical mechanics. Just like GR reduces to classical mechanics in the limit of low masses and small scales. This is an important aspect of the development of scientific models, it is what the word “reduce” means in this context. It means that quantum mechanics is a more encompassing model, and by taking the appropriate limit you can reduce it to recover classical mechanics.

> I don't mean to sound flippant, but that is an awfully convenient excuse. Which is to say that since you cannot demonstrate that you can perform such a feat, it remains conjecture rather than knowledge, faith rather than fact, a valid supposition but not a foregone conclusion, that it actually can be done.

It’s not an excuse. In a Newtonian world, Newtonian mechanics could be used to deterministically predict the precise time evolution of a gaseous system given accurate and precise initial conditions, regardless of the number of gas particles. It is wildly impractical to do that for anything above a small number of particles, but that doesn’t make it conjecture. And again, I’ve already proved your second sentence wrong. You just chose not to even acknowledge it.

> "Often", after all, does not mean 'always', and whether the instant case is such an example requires empirical demonstration or else it is simply not convincing, all the more so because of the specifically confounding results that differentiates QM from classic physics.

Again, I literally gave you the proof. You’re not even just using ignorance as a weapon here, you’re using willful ignorance. QM has been unambiguously and explicitly proven to reduce to classical mechanics in the macroscopic limit. Hard stop.

> And also as I pointed out, it remains an act of faith to suppose that means it is true in reality.

And again, it’s been proven true in reality. The model of QM reduces to the model CM in the macroscopic limit. You can argue all you want against that, but you’d be wrong. The only place “faith” shows up is in the assumption that QM accurately models reality, but I’m not arguing whether it’s correct or not (and also that is always the case for every scientific model that ever has and ever will be constructed, making it a pedantic argument in the first place). The fact that QM reduces to CM is mathematical fact, regardless of the applicability of either model to the real world.

> Then it, quite arguably, has nothing to do with the discussion, in a way very analogous (or perhaps entirely identical) to whether QM "has nothing to do with" actual reality, but only mathematically models particular systems in scientific laboratories.

No, the other person made false claims about the limits of applicability of quantum mechanical systems. They weren’t arguing that we can’t know whether QM is actually, truly correct. They were clearly under the wrong impression that QM, even if true, doesn’t apply at macroscopic scales. That is what I argued against. You are merely creating strawmen.

> But I still insist you are overstating the case of their applicability, since functional utility in restricted examples does not actually prove general accuracy in all instances, even with further theoretical bases to support the belief that QM fully explains why Newtonian physics arises from quantum interactions.

Except I’ve given explicit examples of ways that QM empirically is relevant in macroscopic systems. But as usual, you simply don’t engage with the parts of my arguments that you don’t know how to address and pretend they never happened.