First, the macro level figures provided in the article are only industry best practices and their concrete application will raise field issues such as adapting to local standards, users behaviors, industry habits or externalities. Though it would undoubtedly be much efficient, you can’t use outside isolation for old Paris center buildings. You would also have to train workers to perform meticulous isolation techniques. And the building owners won’t pay for an investment that will mainly benefit to the tenants. Those numerous micro level issues make energy efficiency implementation far more difficult than what it looked from a broader point of view.

Furthermore, even if solutions are implemented at micro level, the rebound effect has to be taken into account. It is indeed uncontroversial that when a technology improves energy efficiency, it entails an additional consumption of products, through substitution or income effect. For instance, thanks to the money you saved with your low energy light bulb, you can now buy an additional light bulb.
Some economists* would then suggest taxing energy efficiency, so that energy prices do not decrease accordingly. In other words, as it is always the case with energies in general, politics has to embrace the issue. However, politicians often tend to spend too much on implementation, and not enough on evaluation and sanctions. Ultimately that’s the reason why energy efficiency is so inefficient.

*On this subject see Perceptual and structural barriers to investing in natural capital: Economics from an ecological footprint perspective, Mathis Wackemagel, William E. Rees
Or Le plein s’il vous plait, Jean-Marc Jancovici