Although if the cup is warm to begin with, then you're using energy to warm up the cup which would eat into those energy savings.
The "optimal" drinking temperature of espresso is still considered higher than 50 degrees, like 60-65 degrees, though of course that is subjective, but the nice aspect of serving it hotter is that those that enjoy it hotter can have it and then it cools quickly and the customer can choose when to enjoy it or get to enjoy it at multiple temps.