I think, as a thought exercise, one can make the same argument about plastic. Even though it is cheap and there is demand it does not mean that it does not have

I think, as a thought exercise, one can make the same argument about plastic. Even though it is cheap and there is demand it does not mean that it does not have also bad consequences (non biodegradable in that case). In the fast fashion case the bad consequences would be slave labour and pollution (from materials and from transport) for garments that do not last (usually fast fashion refers to mediocre quality items), or that are returned immediately.