It seems to me that the wealth process, which is largely fed by the income process, ought to be the primary process in economics for consumption. That process, crescet, diminishes through consumption (and taxes) and grows through investment of some (high) fraction of one's wealth (or all of it, with a fraction assigned to safe cash) and a series of jumps on the wealth process through unexpected spikes and troughs on the wealth process. Rather like Modigliani's lifetime consumption hypothesis, and like Friedman's refinement for non-windfall income, there needs to be a model of the fraction of wealth which is considered appropriate for investment. There may be a much tighter constraint on the wealth process around leverage (and borrowing). But there essentially needs also to be a fraction of wealth which would substitute the need for income borrowing.
As a matter of western world 2018 fact, we're mostly always in a position of borrowing, thanks to the desire embedded in most of us to own our own houses. The next question which arises is, how to model the wealth process mathematically, and how the fraction given over to investment ought to evolve over the lifetime of the individual.
