Firstly, I think you are underestimating the societal impact that would have led a cryptocurrency to be worth $1 trillion. It would mean it has succeeded on a massive scale: foreign trade made more fluid thanks to Bitcoin's disregard of political borders, hundreds of millions of people using it to escape inflation/financial restrictions/etc (vs. today's few million users), millions of merchants accepting it (vs. today's 130 000 merchants). All Bitcoin would have to accomplish to justify using 0.X% of the world's energy is to increase the world GDP by 0.X%.
Secondly, 0.74% is an upper bound. It assumes power costs of $0.01 per kWh (read section 2 at http://blog.zorinaq.com/?e=82) when actually the average power cost worldwide is 10x higher ($0.10/kWh). Maybe it will be 0.3% of the total energy. Maybe 0.1%. How low is low enough that you would consider it worthwhile?