Let's take going to Mars. We can't reliably go to the moon. We can't even go everywhere on the Earth, where we have every possible advantage. Spend a year on the ISS and you'll develop all sorts of health issues. Spending time on Mars isn't likely to be less hazardous. We may send humans to Mars but it is by no means guaranteed. (And I would also ask, what reason do we have to go to Mars? Probes do a better job of exploring, and I doubt we could colonize it.)
Until someone builds a real self-driving car it is just an idea. On today's roads, it is probably not possible to safely implement a fully self driving car. Driving is not only a technical exercise but a social activity that involves communicating your intentions to other humans, and interpreting the intentions of others. That is something that humans do far better than machines. (Edit: To clarify, humans are better at communicating with other humans. Machines do a great job of communicating with each other, but have mixed results communicating with humans.)
If every car was self-driving and the roads were remade from first principles, then sure, that seems feasible. The degenerate case here would be a self-driving train, which seems perfectly reasonable. But the technical challenges are the easy part of that endeavor. Funding such a project, developing the political will to see it through, and organizing the logistics are far more difficult. Consider for instance; what will happen to the legacy vehicles? Will it be illegal to drive them? Will there be a massive government buyback? Who will fund that? Where will the cars go? How will we organize the logistics of moving hundreds of millions of vehicles? How long will that take? What other matters will we need to turn our attention away from to accomplish that task?
A much more likely scenario is that companies continue to come up with partial, ad-hoc solutions, driving gets more automated, ride sharing becomes more popular and car ownership less so, but that humans remain in the loop for the foreseeable future. What happens outside the foreseeable future is something we can't and shouldn't pretend to know with any degree of certainty.
There are a million ways in which we could never go to Mars or build a self-driving car. We could get a better idea for how transit should work, making self driving cars superfluous. We could discover life on Mars, and make the decision that it would be too dangerous for us to visit. The superpowers of the world could go to war with each other, and our infrastructure could be devastated to the point where space travel is impossible. Climate change could drive us to extinction. Something could happen that we cannot predict or imagine, that we have no precedent for, that completely changes our situation and outlook.
Some of these are more likely than others, but the point is that it would be folly to take the future as read. And frankly, a couple of them are more likely than us ever going to Mars.
