Please describe how sunshine alone will sustain this unchanged.

Because you're ignoring the hugely inefficient baseline of how much sunlight we have historically been able to harvest.

From this page, sunlight-to-biomass efficiency of wild plants is less than 1%. For crops specifically bred over thousands of years to grow as much useful biomass as quickly as possible in each season, the number ticks up to 2%. But even that number also fails to account for the periods in which fields lay fallow between seasons, or for crop rotation.

Then, once you have that biomass, converting it into a useful form of fuel for burning, or food for biomechanical energy, has huge efficiency losses along the way.

Meanwhile, solar panels achieve approximately 25% solar efficiency.

That's how we can collect far more energy from the sun today than at any point in the past. Biology is inefficient compared to photovoltaics.