The cost of ethically produced fashion is significantly higher than that which consumers are used to, and more than many will be able or willing to pay. For that reason, it is a privilege market and cannot replace the role of fast fashion in the lives of most consumers.
If Fast Fashion becomes regulated, companies taxed for their carbon use and ordered to use sustainable fibers and dyes, and mandated to pay living wages to their workers, the cost would increase significantly, displacing their key consumer.
Where will this consumer turn in a post-growth scenario? Second hand clothing is and will replace fast fashion.