The proof is really in the fact that it is still happening. Mining must still be financially viable or else people would pull the plug because otherwise they are acting irrationally (or derive benefit from continuing the bitcoin fight at their own detriment, but that is hard to quantify). Maybe for small scale home miners you are priced out of the market.

In terms of sustainability and electricity prices there are numerous operations that are springing up in locations where they are able to minimise their overheads or costs (such as on cooling or actually powering the miners). Places in the swis alps are cold and therefore cooling can be reducing saving money and places near power plants are cheaper meaning your operating expenses are lower.