The problem with talking about smartphones and planes is that they aren't enough. For example, a big portion of energy usage is heating and cooling homes. Are you saying people need to give up AC and die in the summer? Or give up heating and die in the winter? Obviously, we should insulate houses, but like solar panels, that takes fossil fuels.
The transformation of agriculture is inevitable or we will all die.
Similarly homes can be insulated, roofs can be made of heat reflecting materials, trees can be grown to provide shade. We can build cooling centers in cities.
I am precisely saying that we need to prioritize. I would rather survive the summer and not fly far away on holiday than die in the summer.
> Is the plan that everybody becomes a peasant?
Well big cities are a problem, and people working in services definitely rely on a society built on abundant energy.
I don't know exactly what the solution is. What I know is that our society is built on fossil fuel, and not only those are limited, but they are killing us. So we need to remove fossil fuels. Then looking at the numbers, it appears that we can't reasonably hope to replace fossil fuels entirely. Hence we have to use less energy, hence we have to degrow.
I hardly advise reading https://www.amazon.com/World-Without-End-Blain-Christophe-eb..., that's from a well-known french engineer.