Its your equation, and that is exactly what it is - the Demand equation.
So you don't understand the equation. You sure your not really fed economist, not dentist?
I am not assuming anything. If disposable income is at or close to zero, the marginal propensity to spend is at or close to zero. That is what the equation says outright, and its a well known fact in the real world also.
It also says if disposable income is high, then propensity to spend is high, which is why even in the United States, which has the highest per capita expenditure on earth by a country mile, 50% of all consumption is done by the top 10% of household incomes, and the other 90% spend the other 50%.