why I want to replace tractors
Tractors are good for one thing, pulling something that's difficult to move, generally because moving it means displacing soil, turning over the top layer with a plow, slicing it and turning it slightly with a disc, or simply clawing through it with a harrow. They can, of course, be used to pull lighter loads, but their design is driven by the need to apply strain to a tow bar.
Displacing soil (tillage) might be termed the original sin, although overgrazing resulting from large herds of domestic animals moving too slowly/frequently over marginal land predates it. Through excessive aeration, tillage burns through humus (the organic content that, among other things improves the ability of soil to retain water), and exposes the soil surface to wind and water erosion. It also consumes a considerable amount of energy, usually in the form of diesel fuel.
To make matters worse, mechanical tillage works best with the worst cropping practice, monoculture, where a single type of seed is sown over an entire field, effectively all at once, and the crop typically harvested by shearing off everything more than a few inches above ground level. It's a practice that's efficient in terms of the number of man-hours required per land area, but at a terrible cost.
Personally, though, I have another reason for wanting to replace tractors; they're dangerous. I grew up in a farming community, and, of the farmers I knew as a child, two were crushed by overturning tractors (inherently unstable because they're designed for traction), and another was killed by a falling disc section.
So please forgive me if I seem a little too zealous, too much in a hurry to retire a nineteenth century technology and replace it with something not yet available, something so different that it will require a systemic overhaul, one long overdue in my humble opinion.