The original thread question brings up a variety of other questions for me.
If crewed missions to Mars or Venus had been attempted early on, such as the 1960s or 1970s, what would have been the impact on the crews? How would they have handled exposure to the interplanetary radiation environment and possible multi-year flyby missions in zero-g? Remember that decades ago, some of the hazards we now know have to be planned for in planetary missions would instead have come as ugly surprises.
Would the prospect of habitable, colonizable, exploitable nearby planets have been enough to overcome the political and economic currents against space exploration? Apollo was a huge expense for Earth's wealthiest economy as it struggled to pay for increased social spending and the war in Vietnam. Using the same technology to support regular missions to Mars and Venus would have been fantastically expensive. Also, in the absence of some sort of "unobtainium" you're not going to be sending back space galleons loaded with gold from the colonies that will pay for the whole venture.
Perhaps the technical problems would create a willingness to consider the otherwise unacceptable, such as Project Orion. Nuclear pulse propulsion was, and is, a technology in search of a justification for building it. Would life on Mars and Venus have been that justification?
Even if the United States had gone for it, would the Soviet Union have followed suit, or looked at the price tag and balked? Following the Americans in a big push would have risked bankruptcy and social upheaval.
If they do succeed in building Syrtisgrad, what sort of place would it be? Do they fill it with scientists and run the risk of them deciding the Soviet system is best left on Earth, or accept the expense of shuttling herds of KGB minders around the solar system?
Although the US and USSR would clearly be the frontrunners, what about the smaller powers? Would European nations try to offset the loss of their Earthly colonies by getting in on the interplanetary action? It starts to look a bit like Space:1889, a century later.
On a different note, if making Mars habitable requires increasing its mass substantially, what impact would that have on Earth? Either directly, through increased perturbations in our orbit, or indirectly, through possibly perturbing more asteroids into our path? If a bulked-up Mars led to even a slight change in the rate of large impacts on Earth, there might not be a humanity to have a space race.