Your "mature economy" is just less material producing economy, that is higher share of service sector. In essense an accounting trick in playing numbers.Electricity per capita largely peaked around 1990, when the buildout of Nuclear power was halted.
For the broader picture you need to look at energy consumption per capita:
This peaked 50 years ago.
And that's the thing about mature economies - ever increasing inputs of energy becomes uneconomical and energy efficiency wins ground.
Your diagram shows that the peak energy consumption of US was in the 1970s and 1980s, which "coincidentally" was when US had the highest share of world manufacturing industry.
Today, China is number 1 in manufacturing, so is her energy consumption.
There is no secret about being "mature", it is just stopping making physical goods. The physical law is simple, for producing anything material, there must be energy input, the more you produce the more energy input. Efficiency isn't something one can brag about if one is not the number 1 in manufacturing.
BTW, the service sector is just redistributing existing physical goods, it is one dollar in, one dollar out, from your left hand to your right hand, from the poor's hand to the rich banker's hand, nothing new is ever created. Only manufacturing creates. Living in a "mature" economy is NOT a good news for the 90% population.