The first big issue with the idea of homemade soft serve is that a soft serve machine dispenses a product that should be eaten as soon as it is dispensed. Normal home ice cream machines contain coolant that can go a bit below freezing, but not much; commercial ice cream machines aren't all that different, but once they dispense ice cream, it then goes into a sub-zero freezer and the ice cream's temperature drops to around -29°F. You want the temperature to drop as fast as possible, to prevent ice crystals from forming and so that the ice cream maintains its structural integrity at the molecular level. Ice cream is essentially oil (fat), water, milk solids (all the stuff that floats around in milk and makes it milk, not water—proteins and such), emulsifier, and sweetener. None of those components really care for each other, so you freeze 'em quick before they start fighting.