The widespread use of electricity or trains or airplanes or dams or any major paradigm changing invention in human history requires mega engineering projects fo

The widespread use of electricity or trains or airplanes or dams or any major paradigm changing invention in human history requires mega engineering projects for a very simple reason. You have to deliver at scale. I disagree there. All the examples you stated developed gradually starting small and expanding. The first railways appeared in Europe in the 14th century and didn't really take off until the 18th century with the development of the steam engine. Even air travel, which went from the first heavier-than-air flight to first commercial flight in a remarkably short period of time of about 27 years, piggybacked off of earlier lighter-than-air travel that had existed for centuries. In the Napoleonic Wars, balloons were used for surveilling the battle field, and airships appeared in the mid 19th century. People already knew that air travel was commercially viable and militarily useful. Heavier-than-air machines were a major improvement on the concept, not something entirely new. This is a common theme in technology: a primitive form of the technology exists for a long period of time in niches. Then, after a period of time, there's a key invention -- the steam engine for rail travel, the dynamo for electricity, the airplane for air travel, the transistor for computers -- that causes the technology to explode in a short period of time. That key invention is the last piece in the puzzle, not the first. The earlier developments prove the viability of the concept, build up the infrastructure to support it, and justify large investments in developing the key invention. But if you don't look at the history before the key invention, you'd think that computers and electric generators and railroads and aircraft appeared suddenly and had large amounts of capital suddenly invested in them. And that brings us back to my original point: all those technologies had clear advantages over their predecessors. What advantage does an all-solar future have over a mix of sources? Simply getting rid of fossil fuels is not good enough.