The costume philosophy is pretty good. The problem with it today, I will repeat, is that all the costumes are derivative of the past but only a few recognize or admit it. (Much the same is true today in the arts.) In the context of RTW merchandizing of clothes, this lack of self-awareness leads to an annoying presumption of novelty or unique discovery masking a lack of talent in both concept and execution.
I agree with most of what you say in this post, but this last but one paragraph just doesn't appear to follow or make internal sense. Perhaps you can help me.
Are you claiming that designers of RTW clothes in general do not have any knowledge of the history of clothing? If so, I would dispute this as many of them have probably studied it and are almost certainly enthusiasts if not formerly educated in the history of clothing. Or are you talking about the consumers of such clothes here? That might be a more reasonable claim. Secondly, are you claiming that the predominant marketing offer is novelty? If so, I would suggest that is only partially correct, and that particularly in menswear, even fast fashion menswear, the novelty involved is often only specifically related to what has gone immediately before and is almost never presumed to be novelty in terms of the whole history of style (in fact, many references are made to the past, perhaps in a way that is rather too postmodern and eclectic for your liking, but nevertheless those references are frequent). Finally, you seem to be claiming that these things result in a 'lack of talent in concept and execution'. One may indeed argue that many RTW clothes, especially, again, those from the fast fashion end of the spectrum are indeed badly made, but that does not mean that even they are badly designed or even that poor manufacture is necessarily bad execution of a concept - since the concept might be quick, disposable clothes to be worn a handful of times. That maybe ethically problematic especially in environmental terms but it doesn't necessarily have anything to do with talent.