Some degree of industrialization is sustainable. there's no reason we can't have bicycles in the future.
There are a few main things which are not sustainable....feeding 6 billion people using massive petrochemical input, polluting the planet with persisting toxins, paving over farmland and depleting topsoil, and maintaining our level of energy expenditure.
What would be sustainable?
"Organic" agriculture and a mostly vegetarian diet. I don't know how many people the earth can sustainably feed, but it's a lot. Unfortunately, it's probably also a lot less than 6 billion.
Reliance on sustainable energy, using far less energy than we do today. Since we squander huge amounts of energy on trivial activities we can do that. Especially with a smaller population.
Elimination of persisting toxins dioxin, PCBs, etc.
Population reduction through various means...voluntary, incentives, war, disease, famine. Partly we'll choose the methods, and partly they will be visited upon us.