There is a repeated pattern of failed investment in major infrastructure projects, going back to the construction of the canal network in the 1700s; repeated in the “railway mania” of the 1840s and 1850s, and the American transcontinental railroads.
Brunel was a brilliant engineer, but an unmitigated disaster for many of his backers.
TML was ultimately built using funds attracted from minor investors, few of whom ever saw their money again, let alone a return. Cornish mining enriched a few, and impoverished many, and that wasn’t unusual.
Major infrastructure projects have a habit of ending under the control of a later wave of investors, after successive earlier investors bankrupt themselves laying the foundations.