Among other things, modernity is about consumption, mass production, disposability, and at at least a partial nod to egalitarianism. These aesthetics are best captured by RTW designers who produce seasonal clothing and disposable "looks." It's this premise I disagree with. It sounds like you are describing one significant aspect of the socio-historical phenomenon of modernism, not what is fundamentally modern. While modernism led to innovations in disposable goods that could be mass produced, it is not defined by such things. Rather, modernism is the drive to dispose of what is needlessly assumed in favor of what is objectively superior. This can lead to both disposable things and things of permanence--and it doesn't always mean newer and more contemporary. When rationality and science don't reveal something superior to what's already being done, a good modernist may stick to tradition and rely on the wisdom of experience. Twentieth century technnology gave us all sorts of advances in synthetic fabrics and mass production techniques, but how many people think poly-blends are more comfortable than high quality pure cotton? A nylon windbreaker has certain advantages over a tweed jacket, but I am not convinced it is objectively superior. An argument can be made that synthetic materials and mass production offer efficiency that traditional tailored clothing can't, but the overall benefits may not be greater. Whether that matters to you is a question of what kind of modernist you are, not whether you are one. It's the post-modernist that is more likely to value the contemporary for the sake of it being contemporary.