Is green the new black or is sustainability just another fashion trend?

I think that the answer to your question is "yes" and "yes." If the fashion industry were to truly embrace sustainability and acknowledge that there are already enough clothes and accessories bouncing around in the world, then it would seize to exist. Ideally, they should be promoting the notion of recycling/swapping/reusing what is already in circulation and no longer go back to the drawing board in an attempt to create the next new trend that will translate into lucrative sales. If the industry were to create fashion from this point forward that was exclusively made out of 100% recycled waste, that would be a very sustainable effort worth the public backing, but I find it hard to imagine that it will ever come to that. Companies want to stay in business no matter what is going on in the world around us, so that's partially why so many of them are jumping on the green bandwagon. There's a reason why the term "greenwashing" was created.