What will happen when advertizers skip promoting green and move on to whatever sells stuff next? I think this depends on how long particular green issues can stay in the public mind. Overall, I'm not too worried about a 'backlash' against the current greening of the economy world provided some current trends actually last long enough to become the new norm.
Most people never used to think about the 'green' properties (low energy use, sustainable sourcing, recyclable) of what they bought. Now many do. So the question becomes whether the support for 'green' is becoming sufficient to make such considerations a default part of product development, even when the actual marketing has moved on to other things. If so, then even if selecting a green product is no longer trendy, it doesn't have to become actively uncool, but merely normal. Green factors, even when not be top of the list for purchase choices or marketing, can still be influential if customers expect 'green' and so penalize companies/products that fall below their new (higher) standards.
With green behaviors of products there is a long lifecycle to reach widespread adoption; crazy ideas become cool, then normal. For things like organic food or wind and solar power, few early adopters were often decades ahead of everyone else (and characterized not as cool folk, but as crazy hippies). Some environmental ideas break out of this hippie ghetto and gain a wider but still minority acceptance, thereby becoming trendy or rather cutting edge. Eventually, a few products or ideas become popular enough that they can no longer be seen as trendy but instead are commonplace. There are a few green choices that reach the 'normal' level yet, but recycling has become widespread enough to be normal in many areas. Low energy use products stand a good chance of taking hold, I think, as there is a personal economic advantage along with the global benefits.Young people in particular express a strong desire to 'save the planet', so the prospects of more green choices increasingly becoming normal are good. A danger is the desire to cash in on this youthful goodwill. There is lots of greenwash out there (products overselling limited or dubious environmental benefits). There is thus a danger that they will taint the whole green movement as people tire of false green claims and disregard the behaviors (and even products) that can deliver real environmental benefits. With so much commerical mis-information and so many everyday choices to make the public can only make sensible informed decisions if there are reliable and measurable standards of 'greenness'. Vigilant denouncing of greenwash can also keep corporations honest. Current high public and commerical support for 'green' choices can yield lasting change if those choices provide real envirnomental benefits and become common enough to take hold. Indeed, no longer being a fad can be a good outcome.