By taking away a portion of a year's profit from McDonalds, you've taken away a portion of investment gains of the following year:

If I grows by 3%, then taking away 20% of I affects growth by .2I*.03 = .06I. In that same time span, the increase in C is offset by the decrease in I, so in effect you've only decreased total production Y by .06I. Any benefits of increased demand will be seen in the next year after the new profits of McDonalds from an expanded customer base. But there would have been more capital to use to invest had you just kept the money in I in the first place.

tl;dr money is naturally already siphoned into industries that actually improve our conditions, with the exception of useless shit like yacht production and the like. By putting it in the hands of people who dont actually contribute, you're taking away from better sources of growth.