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-transactionYes I would like to see it as a percentage, but remember BTC has already established a president of LOWERING MINIMUM FEES to keep fees affordable as the value of BTC rises. That is continually pushing back the point when the Network can sustain itself by transaction fees because it counteracts the rise in revenue that miners would normally see. I would not be at all surprised if Fees are actually dropping as a percentage of mining revenue. By my Math the transaction fees are 0.6% of miners revenue now, but I lack the data to establish a trend, I can see that transactions measured in coins has been fairly constant at 25-50 per day for a year, this seems to be what people are willing to spend and what miners are willing to take. If that number holds then we would see Transaction being about half of miners revenue in ~30 years, but I expect a continuing decline in fees such that this point is pushed out 1 year per year, a messiah that people will continually be withing for.