Thanks for thinking through this in more detail! I think I roughly agree with you for the cases where someone is already good at singing and/or playing an instrument; I do think musical skill generalizes fairly well. (And thanks for spelling out the scenario of “you can actually practice this hands-on intensively for weeks or months in a row as your primary activity”; my default felt sense of weeks/months necessary to do something does not by default involve total primary focus.)
I don’t think I expect non-musicians to be reliably able to skill up in 6 focused months, though! Some would, if they have a good ear and, say, facility in learning languages or similar (to get really used to sheet music). But some would not be able to pick up basic pitch manipulation fast enough, and some might get stuck on becoming fluent in sheet music, and some might have trouble with the kind of vertical listening needed to evaluate harmonies & pick up mistakes.
Though to be fair, the part of that I’m most confident of is that it’s very hard to get from “tone-deaf” to “proficient with sufficient ease to scaffold additional complexity”; I might be more willing to believe that if you already have that kind of proficiency you can often skill up significantly in 6 months if you focus fully on it.
I’m curious whether e.g. Democrats mostly thought it was a Republican thing and Republicans thought it was a Democrat thing, or if most people thought it was their own party’s thing, or neither