Why? Given the chance of injury and short careers owners would have to offer less and shorter contracts to reduce the risk that they will be paying players who no longer are playing. If they have to do that, the rational economic response is spread out the money you would pay in one year over several and offer performance bonuses equal to what you'd pay over the life of a non-guaranteed contract. We already know what the owners are willing to pay so the question becomes how do they structure those payments if contracts are guaranteed?
As for a minor league system, they already have one they don't have to pay for - college football. They assume none of the risk of injury, long term health impact; and do not have to pay anything to have very talented people training kids for the league. All they have to do is pick the ones they want and not incur any obligations to those they don't pick.