Sure we have several possible replacement technologies, but the change of underlying infrastructure requires a long lead time. Solar energy is abundant and present. Every second, the sun converts 600 million tons of hydrogen to helium, with mind boggling amounts of energy released. The sun is so big it takes thousands of years for that energy to filter out to the surface where most of it is released as light. This is a huge amount of energy in practice, quite enough for humanity's current needs, but insignificant compared to what is available in space, even in near earth orbits.
Abundant energy means abundant wealth.
With basic needs met on earth, and a new frontier in space, there is a chance that the earth will become a peaceful place. Unless these needs are met, war will be common, and if we remain reliant on fossil fuels, those wars will very soon affect everyone. Looking at the history of humanity, many other reasons support this approach. For my 40th birthday party I organised a possibility party. About 20 of us spent the day brainstorming threats to humanity, and possible responses.