Because in the US, a tractor is considered an tool, and thus the farmers/farms get a 100% write off of the entire purchase price.

A tax credit is not the same thing as a deduction (write off). A credit goes dollar-for-dollar against taxes owed. A deduction reduces the amount to which the tax percentage is applied.

The used market is pretty good.

But you need to declare the sale as capital gains.

I also don't understand this argument. I use computers for my business. It makes it cheaper to get one for business than for personal use, but I'd still like the things to last as long as possible.

A PC is different because there isn't much correlation between age and operating costs.

With something like a server, as long as you can deploy them quickly, you actually can reduce costs by replacing them in step with the Intel tick/tock cadence.

Only for as long as the DRM key-holders allow it to be

I suspect they run the used asset (say worth $70k) through some sleight of hand with subsidiaries or leasing agents to avoid the depreciation recapture tax of selling it for its true worth. Voila, free tractor.