Implementing sustainability in a well-established fashion company can be a complex and challenging process. It often requires significant changes to existing business models, supply chains, and production processes, which can be difficult to implement and can take time to see results.

On the other hand, starting a new sustainable brand allows for greater flexibility and the ability to design sustainable practices into the business model from the beginning. However, it also comes with its own set of challenges such as, building brand awareness, developing supply chains and distribution channels, and competing with established brands.

That being said, both options have their own set of challenges and opportunities. A well-established company might have more resources, knowledge, and experience but also more legacy issues to deal with. A new sustainable brand might have more flexibility but also less awareness, resources, and experience.

Ultimately, it's important for companies to evaluate their unique circumstances and determine the best approach for them. Many well-established companies have succeeded in becoming more sustainable, while new sustainable brands are also emerging and gaining popularity.