Life without fossil fuels
When someone says they want a “sustainable future” where there are no more fossil fuels, they envision one of two futures: 1) fantasy land or 2) a more primitive civilization.
Our civilization was built on and made possible by ever-increasing energy density. The importance of more energy-dense materials is they provide a higher energy return per unit of energy invested/expended (“EROI”). All creatures (and civilizations) must acquire more energy from hunting and gathering than they expend hunting and gathering, or they soon die. So, the animal must have at least a 1:1 ratio of energy expended to energy gathered. Further, animals (and civilizations) also require shelter, so they must gather exponentially more energy than they use to simply hunt, gather and eat. Still further, they need an even higher amount of energy to defend themselves from predators. Even if the animal (or civilization) gets enough energy to gather food, find shelter, and defend itself, without reproduction and protection of its young, its species (civilization) will go extinct.
So, keeping your species from going extinct requires even greater multiples of the energy expended to cover all your other needs. This is all the bear minimum of survival and does not include growing your population/civilization, having an educational system, court system, bureaucracy, healthcare, art, the internet, sports, etc. In other words, a modern society.
To have a modern society, the multiple of energy returned on energy invested must be over 12 times, and depending on how sophisticated a civilization you want, it must be over 20 times. There are plenty of academic studies backing up these multiples.
For context, an all-in analysis of the energy returned per unit of energy expended puts solar energy at an EROI (energy return/energy invested) of 3:1 at best. In short, it cannot sustain a modern civilization. Maybe a new technology will emerge that can actually replace fossil fuels, but dealing with reality those are the options: Return to a primitive society or use fossil fuels.