Modern combines sit idle when the harvesting period ends. So about 90% of the year. With indoor farming, you are not constrained by the seasons. You can rotate your crops, so you harvest some in January, some others in February, etc. Whatever machines you need for harvesting, spraying, tilling, etc, you can arrange to use each 100% of the time.
I personally think grains will not be economical indoors this decade, or maybe even next, but in some not very distant future, the majority of the agricultural products will be produced indoors, just like chickens are now. Of course, when that time will come, indoor farming will be perceived as bad, horrendous, anti-nature, etc, just like people perceive the industrialized chicken farms now.
Neat thing about idle equipment, it doesn't consume any inputs, so not real sure where you think you're going with that one. And yes, you aren't constrained by seasons. You're constrained by grid electricity rates, market prices for agricultural products, available labor pool, and the supply of dumb money naive enough to capitalize infrastructure projects that are balanced on that house of cards.
I personally think grains will not be economical indoors this decade, or maybe even next, but in some not very distant future, the majority of the agricultural products will be produced indoors, just like chickens are now. Of course, when that time will come, indoor farming will be perceived as bad, horrendous, anti-nature, etc, just like people perceive the industrialized chicken farms now.