Good post! I think we'll have to define warmth to be more precise. Most think of warmth as an addition to the sound that is similar in character to even order harmonic distortion. At mid and high frequencies it indeed sounds subjectively warm, but this addition smears or hides detail, the effect is adding a "sameness" to every recording that makes timbre less distinct and reduces the spatial performance of the system. At low frequencies it "fills out" bass and again, sounds somewhat similar to harmonic distortion. This reduces the sharpness of transients and rounds off the leading edges of dynamic swings, and again reduces the distinct timbre of the instrument or vocal. It's not exactly the same as harmonic distortion or speaker/room acoustic issues, but there are definite similarities in how it's perceived.
Some examples... gold plating adds warmth that smears the sound and is the primary reason most high end connectors are not gold plated. You can't buy top-end Furutech in anything other than rhodium and there's a very good reason for this. The issue is most won't go "all-in" on using rhodium and the combination of rhodium with other plating materials is what causes problems and what some think of "the sound of rhodium". Many of my customers have gone all-in on using Furutech rhodium AC power products and in every case the artifacts described as the sound of rhodium simply go away. But warmth does go away too, for better or worse depending on the system, recordings listened to and personal preference. But if the goal is "high fidelity", imo gold plating has no place in a HiFi system.
Copper adds warmth. How much varies, and UPOCC copper is noticeably clearer and less warm than other kinds of copper. High voltage treatments can reduce warmth and increase clarity too. This is my primart criticism of commodity level copper cables like Belden, Mogami, etc... it's warm and smears/hides detail. It prevents a 3-D immersive soundstage from forming, which is imo the primary driver of subjective preference, and thus it fails to provide a "you are there" experience. Instead, you get the much more typical sound of the performance happening in your listening room, and a sameness to the spatial presentation across recordings. Again, if the goal is high fidelity, most copper cables have no place in a HiFi system, but there are some exceptions. OTOH, the best UPOCC silver is still better as far as clarity and resolution than any copper cable. UPOCC silver is expensive, so I always recommend prioritizing interconnect cables. It's much easier to get good results from copper PC and SC vs IC cables.
A certain amount of warmth is required to have realistic tonality though, but a lot of this is ideally gained from the reduction of noise that causes the subjective effect of brightness. The more of this kind of noise you have, the more warmth is required to balance it out and hide these flaws and prevent listening fatigue. Some things can add warmth without as much sacrifice in resolution as others too. I have a new cable that I've been meaning to introduce for a long time, but covid-related health issues have prevented that. The cable's main goal is a reduction of noise that is typical with conventional wire, and the end result is a total elimination of any hint of "the sound of silver" and a total lack of brightness. OTOH, it's not warm either, It's just very quiet, incredibly resolving, but it's also very low on stimulation as a result, so it doesn't sound as exciting as some might like. This may not be preferred on rock or electronica, but for acoustic music and very complex large-scale classical, it's on another level vs any other cable on the market.
Also, I 100% agree a good cable will lower the noise floor, but that is with many types of noise and artifacts, warmth is only one of many issues a good cable avoids adding to the sound.