No source of alternative energy can ever replace fossil fuels, renewable or otherwise. The deposits of oil, coal and natural gas we are consuming as fast as possible were created over tens and even hundreds of millions of years. Once they are gone, they are gone. However, this doesn't mean that renewable, sustainable forms of energy, such as wind, solar or geothermal, cannot serve an important role.
The first mistake is thinking that we can just replace fossil fuels with a collection of alternatives, without otherwise changing how we do things. But fossil fuels essentially represent millions of years of stored sunlight, and no other energy source can even come close to the energy density they provide. If we could store up millions of years of wind energy (or solar, or tidal) we might have something comparable. But this energy was clearly not stored, and we need energy today, not millions of years in the future.
The second mistake is thinking that we should even try. It is precisely our massive use of energy, and our giant industrial economy, that is causing massive environmental damage. We are already over carrying capacity, so we need to move towards repairing the environment, not further damaging it and depleting our natural resources. The last thing we need is more overfishing, more deforestation, more soil degradation, and the continued paving over of natural habitats. We need less economic growth, not more.
But, just because alternatives such as wind or solar cannot allow us to continue business as usual, that doesn't make them useless, indeed this should be considered an advantage. If we can dramatically scale back our consumption and waste, and switch to more efficient transportation methods, they can provide enough energy to allow us to live healthy, comfortable, happy lives. They only thing they cannot do is to allow us to live extravagant and wasteful lives.
Alternative energies, even if widely deployed, will never provide enough energy to maintain a transportation system based on cars and trucks. Even if we switch to electric cars, we could not produce enough electricity to charge hundreds of millions of them. There is also the issue of maintaining an enormous road system, which requires a great deal of energy and resources. The widespread use of rail as an alternative, both for cargo and personal transportation, however, would require substantially less energy. Travel and tourism would also need to be reduced, especially airline travel, which isn't possible without fossil fuels anyway.
At the local level, cities need to be designed in a more sustainable way, and many experts and city planners are already working hard on this. Walkable communities would become the norm, with an intermixing of residential and commercial areas, so that many fewer people need to commute. Instead of sprawling suburbs, with large houses, each with their own lawn, residential areas would be much more densely packed, with smaller and more efficient homes. With good insulation, and the use of passive solar, residences in many areas can get by with almost no active heating or cooling.
With the elimination of most cars and trucks, efficient and dense housing, and massively reduced consumption, it begins to become possible to envision the practical use of alternative energies. Rather than building millions of wind turbines, or covering entire deserts with solar panels, relatively small scale deployments of alternative energy generation systems across a country can serve to meet people's needs. We would have enough energy to keep ourselves warm, cook our food, keep the lights on, run local hospitals, and power other vital services.
Certainly, our way of life would be different from today. We would be living simpler lives, with less gadgets and electronic toys, and other material goods. But we would also likely have less stress and more free time. We are not talking about the stone age here. By decreasing industrial activity and the production of useless or disposable goods, we would also be decreasing pollution and improving the environment, which would help to improve our health.
Alternative energies can never be deployed as a plug-in replacement for oil and coal. They can't match our current energy demand, but it is precisely our current energy demand that is destroying the planet. However, alternative energy can provide more than enough power to meet our basics needs, and even to ensure our health and comfort. It is possible to live sustainably, to live within our means and the means of our planet, and still live happy and fulfilled lives.