Doesn’t work > see thermodynamics.
The mirror you’d be setting up would have to set up away from the building where the food is grown and therefore occupy space.
In agronomy there is a concept that the rate of growth is set by whichever one necessary nutrient is limited. Since we fertilize and pump water, the limiting factor is sunlight.
EDIT: 1. This is why Brazil gasohol is the only net positive bio-fuel
2. Manual picking of produce is far superior to mechanized harvest. Tomatoes in the US are large and tasteless because they’ve been bread to be easily picked by machine. ( A lot of thought goes into picking a ripe fruit. It’s not obvious AI can do that. Or that the robots can ever grab fruit with dexterity and w/out crushing)
3.What energy source do you want to run your automatic tractors off of? Diesel for agriculture is arguably the segment that should be given the most slack when transitioning to “green” alternatives.
(Sure, you'd still want batteries to drive the machinery between fields, and you'd need a way to keep the cords from damaging crops or disrupting the plowing when you're dragging them around, and you'd need to put a huge "tractor electric outlet" by every cluster of fields you're working with, but these are all engineering challenges that could be met with current technology.)
Vertical farming is unsustainable because then you will have to cover 10x the land in solar panels or use fossil fuels.
I think direct nutrient synthesis probably has potential, but it will take a very long time to understand nutrition and synthesis of organic mollecules -- not an expert by any means, but I'd be surprised if it took less than several decades to develop the necessary tech/science. Even then the gains in efficiency will probably not be that large -- plants/life in general have been optimizing synthesis for billions of years, they're already quite efficient. What we would gain is minimizing to its limit parts of the plant we don't consume, and concentrate on the essentials/optimal nutrients.
I do feel the same about the mechanical side of farming. I'm sure there's room for different tools and structure.
It also surprised me how old school modern farming still remains. Look at the first picture from this article: a bunch of modern tractors unnecessarily burning oil, unnecessarily operated by humans. By now this should be three times the size field with 15 levels up, run by 3D seeds/soil/water/plants/etc sampling throw out from automatically replaceable cartridge/dispenser.
We have long way to go, but certainly we don't need more land to grow; we need more technology in place!
Edit: should have mentioned it - even going vertical you can still use natural light! Its a matter of setting up bunch of mirrors reflecting sunlight. Of course you need sun to grow.