Fast fashion and luxury brands will have different approaches to tacking sustainability issues purely based on their revenue model. This is because fast fashion will have lower margins and enjoy less price elasticity, so despite having economies of scale, there may be a practical limit to what these companies are willing or able to spend on reducing their own environmental impact as a company, including the impact of the products they sell. Nevertheless, concern over the environment is not a passing fad, so these companies will need to prioritise this issue – not least because consumers are becoming increasingly discerning and demanding about the environmental credentials of the companies they support and the products they buy.
Luxury brands usually have less power in terms of economies of scale, as they deal in lower volumes. However, they often have greater price elasticity, so they are better able to do what needs to be done when it comes to sourcing higher quality and more sustainable materials, improving traceability of those materials, and so on, and increasing the price of their products as necessary to achieve their goals. The other benefit of high-end products is that they are designed and built to last, they are more likely to be well cared for during their lifetime, and are much more likely to be repaired when the need arises as opposed to being thrown away at the first sign of wear. They can even increase in value over time, and can be kept for many decades and passed on.
It's also important to keep in mind that where high end fashion leads, the rest of the market tends to follow. So, luxury fashion brands can keep flying the flag and showing that sustainability is at the top of their agenda, and this should have a trickledown effect on the whole fashion sector.