I also don't even know if saying that fast fashion is bad because the cheap, poorly made goods don't last very long and therefore the consumers purchase more of them is even a legitimate argument. Couldn't you easily say that the artisan slaving away over a single pair of $3,000 boots is more 'wasteful' in terms of labor spent producing a pair of shoes that only one person can use?
The root problem with his article is that he's trying to defend what's really indefensible. His site (and now that he has a magazine, his business) is premised on the idea that there exists a small group of educated and thus truly fashionable consumers who are beyond trends, and that cost is one of the barriers to entry to join this group. That's fine, but trends clearly influence this small subset as much as they do any other group - artisan products themselves are a trend - and price is clearly, as Fok pointed out, not a gateway to a more knowledgeable consumer.
You could make an argument that the collaboration is somehow betraying the Margiela's original design vision, but he chose to sell his brand, so I don't have a lot of sympathy for that argument.
Marketing buzzwords aside, the democratization of fashion is a real thing, although it has much more to do with the internet than H&M and Zara. Look at someone like Tavi Gevinson - she's a teenager who, through the internet, was able to interact with fashion luminaries. The walls between "consumer" and "fashion insider" are starting to break down, which is a great thing in my eyes.