If "disposable design" were the beginning and end to the Ikea / H & M business model, then I would think they would have some reason for long term concern.

However, their hallmarks (and that of Zara as well), run more along the line of being very good at sensing consumer preference in the goods they produce and then being just as well positioned to design, produce and bring to market products that reflect those consumer tastes.

If green is an emerging consumer preference, I would suggest that they would be better suited to adapt than a majority of their peers.