The strategic planning of long term projects isn't solely an engineering problem but also involves business, financial and political planning.
To take the example of renewable energy and to generalise somewhat.
First the technology needs to be developed and proven. This is usually an iterative process, initially on a relatively small scale. You first have prototypes then pilot schemes then full commercial roll-out Infrastructure technology often involves a degree of collaboration between business, technology and government.
For example government may stimulate demand by offering grants, tax breaks etc or by legislating to reduce the attractiveness of alternatives. For example if renewable energy is taxed at a lower rate than coal power, then that makes renewable a more attractive investment proposition.
Any new technology is a business risk and investors will calculate whether the potential return on investment justifies that risk. This calculation will involve both engineering feasibility studies and economic modelling to try to predict future economic conditions.
Equally while you can't completely control future markets you can trade in them, energy wholesale contracts may be agreed decades in advance and often energy infrastructure projects will include complex contracts which set pricing structures.