H&M and other fast-fashion companies that focus on never-ending expansion, going on 2,800+ retail stores and counting, can't ever truly be sustainable unless they operate in a perfect, circular system in which everything they take out is put back in. Their business model is founded upon mass production, repeated, mass consumption and designing pieces that fall out of fashion within two weeks. This model has to change before mainstream fashion can ever be sustainable.
We shouldn't be trying to lower the price of sustainable fashion to fast-fashion levels at all. The prices that we've grown accustomed to are only low because we rely on cheap labor and cheap material to produce them. What we should be focusing on is educating consumers and changing their perspective of fashion.
It's also quite archaic to assume that fashion, how it's designed and consumed, won't be changing in the coming years. This change might mean a natural progression to more sustainable practices. For example, many predict that within a few years, most households will have their own 3-D printers. Imagine how this'll change fashion consumption. Instead of designing and making clothes, designers will write programs and sell patterns to customers who will make their clothing right at home.
Think of innovative textiles. Imagine fabric that can be recycled again and again, or fabric that can change shape, color and texture at the whim of the wearer. There are so many opportunities coming our way and I think sustainability will always be a part of the equation.