As someone who's introduced a completely new material to the accessory market, my two cents: The market is full, certainly, but there's always innovations to be made, and the market rewards innovation.
No one was using Richlite or any layered paper composite in any application beyond guitar bridges and fingerboards at the scale of an iPhone case, especially with the small features of our case design. We were the first (October 2011), as as a result, showed other companies that it could be done. They're still figuring out that the case's strength comes from Richlite, and not from bamboo.
Notably, we're the only maker of all natural material composite cases for the iPhone 4/4S.
We're very aware of new, sustainable materials, and I personally think that there will be more emphasis on what these 'throwaway' products do in the waste stream. We'll see bio-degradable plastics, and perhaps more regard to durability, and maybe even repairability.
There's always going to be new materials that perform better and make old ones obsolete, and therefore will always be new accessories. Even if Apple makes a indestructible iPhone, there will always be those who want a custom aesthetic, and will look for accessories that provide it.
So, yeah. Case-making is a tough biz, with lots of players. I've seen several come and go since we started, but they were doing what many others were doing and no better. Innovation keeps you relevant, and keeps you in business.
