If some investors will not always be willing, then there is an implied second group of investors-the remainder-and what is their view? The uncertainty detracts from the clarity and force of your point.

Fixing it could be as simple as dropping the "some":

It is a fundamental mistake to assume that investors will always be willing to provide the capital for any project with a positive rate of return.

Or (which might or might not be closer to your original meaning):

It is a fundamental mistake to assume that an investor can always be found to provide the capital for any project with a positive rate of return.

??