What makes bitcoin unsustainable? The fact that it will some day stop rising in value?
No. Reread Impaler's post. What is unsustainable is that transaction fees are rising rapidly, but that at the same time they are unable to cover network costs.
But - the second graph doesn't mean a lot by itself, we need to see transaction fees as a percentage, not as a dollar figure- and even then what's interesting is the median, not the average.
https://blockchain.info/charts/cost-per-transactionThe transaction fee has been adjusted several times already and will be again. And participation in the network is voluntary - if it's too expensive for you, you have the option of not being part of it. Most objections of that sort stem from simple ignorance of some aspects of the system. And I have no doubt people will keep making them for years to come.