Sure, all economic activity is measured at the margin, but all businesses depend on the marginal consumer pushing them into profitability to survive. On the other hand, most businesses function perfectly happily without any secondary market investors or any realistic prospect of selling on the secondary market in the foreseeable future. Absence of secondary market liquidity reduces the sources of investment available to growing businesses, but doesn't drive it to zero, and thus the real economy tends to be much less sensitive to the marginal dollar invested in blue chip stocks than the marginal dollar consumed.

Perhaps even more importantly, I also pointed out that (i) the marginal contribution of buying secondary market stocks to the growth of the economy is substantially less than their average 7% annual return for the passive investor and (ii) because the risk adjusted return of this economic rent is an attractive alternative to investing in creating new capital, it can in certain circumstances even have a negative effect on economic growth. Those were the real points Keynes was trying to make.