A reader writes:

Let us also bear in mind that in almost the entire Muslim world, capitalism and colonialism (in its various guises) have been inextricably linked. While capitalism should (theoretically) open doors to all peoples, for the last 150 years, it's been a vehicle to transfer wealth from to western nations and the economic elites who collaborate with western governments and corporations (See Saud, House of). The link between capitalism and political repression whether in the form of direct colonial rule (North Africa, India) or rule by western allies (Iran, Saudi Arabia, Iraq) is not one that is easily broken.

Another writes:

There are two ways of responding to this question.

One is empirical: currently, one of the fastest-growing economies in the world, if not the fastest of all, is Turkey, to the credit of a government with Islamic roots, and a new pious middle class from the socially conservative central provinces. This development has actually been compared to the Protestant roots of capitalism. Not only that: Above one of the gates of Istanbul's Grand Bazaar, an early modern temple of commerce if I ever saw one, is an inscription: He who gains is a friend of God (al-kasib habib Allah). The same text is found on a panel in the Blue Mosque. So you have praise for economic activity at the heart of an Islamic society. The movement of Fethullah Gülen is another example, and it would be hard to argue that for these people capitalist activity clashes with their religiosity.

Sorman's summary of the argument about waqf does not hold water (I haven't read Kuran's book yet, so I don't know if Sorman accurately represents his position). How is running waqfs like family trusts, and in the process depriving the central government of revenue, indicative of a lack of capitalist institutions or spirit? If any, it proves that Muslim entrepreneurs knew how to use the existing legal framework for capitalist enterprises.

Which brings me to the second point, that the question as such has to be questioned. This type of essentialist argument about Islam as a unified whole obviously does not make sense, as has already argued in an article you linked to earlier, and as is evident from the examples above. Therefore, the observation that Islam was founded by a merchant is entirely meaningless, since there is no essence based on this origin. Just as the Bible was able to accommodate countless different and contradictory interpretations over time, do does the Koran. Islam is practiced in myriads of ways, and these ways have been conditioned by historical, social, cultural, and political factors.  In math, if you try to divide by zero, you are punished by meaningless results. In history, if you ask ahistorical questions, you are punished by meaningless debates.