What do you mean when you ask, sustainable? No system is without losses so every system is unsustainable over the long run. Even the Sun will run out of fuel, someday.

In order to be sustainable, consumption must be managed. If sustainable means just to avoid collapse. If sustainable is enough energy so that 9 billion people can live much like they currently live, there is no sustainable energy system. If one hundred years ago the population stopped growing, the fossil fuel economy would have been sustainable and would not have led to the current environmental disaster.

A sustainable future could be achieved with an elimination of wasteful consumption, zero population growth and a more equitable distribution of scarce resources with the technology we have today, and it would allow the environment to recover.

We are going to have a disaster because we delayed the transition too long and we are not transitioning fast enough. There will be a "new dark age" depression because we did not work hard enough to avoid it. Coming out the other side will be a "New Renaissance."

I wish I knew when the depression will hit and how long it will last but those cards are not yet dealt, and we don't know how they will fall.

TEK