Bitcoin is a horrible waste of resources.
When I first was introduced to Bitcoin, I was casually optimistic. I even spent a couple of weeks trying to mine a block chain. But in the end, I had zero bitcoins, an electricity bill a few dollars above average, and burned through some hardware lifetime. In this I found the hard truth about cryptocurrencies: they are inherantly wasteful to an unsustainable degree. The global power grid could not support Bitcoin at the distribution level of the dollar. However newer cyrptocurrencies are being explored, like Gridcoin, that actually attempt to contribute to distributed computing projects in a meaningful way.
After being disappointed with Bitcoin, this became an ideal for where cryptocurrencies could end up. A currency that folds proteins or searches for intelligent life is certainly more sustainable than one which carelessly contributes to global climate change. Unfortunately, it is highly unlikely there will be a progressive cryptocurrency which directly addresses real problems with a mining mechanism. Currencies are difficult to get right and cryptocurrencies much more so. But at the very least, cryptocurrencies like Gridcoin are an excellent step in the direction of incentivized computational contribution to human advancement.