If you transition to carbon-free sources of energy without adding efficiency, energy as percentage of total GDP increases — carbon-free sources of energy still cost (on average) more than carbon-emitting ones. This leaves less for everything else (food, clothing, shelter, medical care).
Sufficient efficiency improvements let us phase in non-fossil-fuel sources at no net cost. If we increase GDP per unit of energy, we can pay more for that energy.
To paraphrase Amory Lovins: We don’t burn fuel for its own sake; we want warm toes and cold beer.
In homes, for example, if efficient use of power can still run appliances and provide heat and light, we sacrifice nothing and save money. That money will pay for more expensive clean energy. The price per kWh will be higher, but the electricity bill will be the same.
Even at high prices, the potential of renewable energy is great — more than we are likely to need this century. Still, efficiency would let us take advantage of costlier sources without economic damage.