Not all reduction of consumption decreases output. For example, leaks could be plugged, insulation can be improved, more efficient machines could be used.
Also, not all output is equal, not does all output have to be done at a fixed time. Heating homes has to be done during winter, but some factories can postpone production a few months to wait for more favourable circumstances.
Furthermore, it's only a reduction in a few energy sources that are required, with gas being the most important. Other sources could be used to compensate to an extent. We're seeing some gas power plants moving back to coal for example. Not great for carbon emissions, but it has more and more flexible sources.
If you look at the current plans in most countries, they're largely about reducing waste and excessive usage. I mean... you could consider not being allowed to heat your private swimming pool "impoverishment", but probably only if you were comically wealthy and entitled.
(I'm actually a little surprised just _how_ un-aggressive the plans generally are.)