Interstellar Travel: Considering the Unthinkable

The majority of space science fiction is based on an assumption: Interstellar travel is possible. Either a way to travel faster than light is possible, or there is a way to side-step the necessity by taking a short-cut, usually through another dimension.
Like many sci-fi fans, I hope we do reach the stars. But if I am brutally honest, to the best of my knowledge, there is currently no evidence that interstellar travel is possible. Wormholes and tachyons remain theoretical. Quantum tunnelling has been observed, but there is no evidence that a human or starship can do the same as a sub-atomic particle. 

Suppose practical interstellar travel is not possible? Rather than a great ocean, the galaxy is really closer to a desert of tiny puddles. Humans are minnows in that puddle. They can thrive and occupy the whole puddle, or shelter below a familiar weed, but they cannot leave the puddle, and nothing can cross the desert from another puddle.
This may be the solution to the “Fermi Paradox”. We cannot detect interstellar visitors or civilizations for the simple reason that they cannot exist.
There remains “impractical” interstellar travel. We could reach some of the neighbouring star systems using “slowboats”. if we are willing to accept voyages of several years duration. Hibernation or generation ships may be needed. By these means, human colonies around other stars would be possible, but very little interaction between them would be practical. A distress call from a colony might take more than a decade to receive an answer. Shipping people to the colonies would have minimal effect on any population problems the Solar system is likely to have.
We could use unmanned probes to explore other systems, although we may not hear from then until a future generation of scientists. Many may challenge the expense of exploring worlds that we can never be visited.
In most near-future sci-fi interstellar travel is not an option, but what happens if this never changes? In a few thousand years, how will humanity fare if it cannot expand outside its home star system? What if it can only utilize the finite space of the solar system? Eight(-ish) planets and various moons and lesser bodies. Some of the Rho-class planets and moons may be capable of being terraformed. For environments that cannot be adapted, ingenious solutions may be needed. Several science fiction novels have track-mounted communities on Mercury that constantly move to stay in the twilight band. Humans themselves may be adapted to certain environments, such a oceans or free-fall. A future solar system may have several sub-species, or even several distinct species of humanity.
Life may be trapped in its star system of origin, but that does not mean that there is no life around other stars. Perhaps we will one day make contact with other civilizations. We cannot visit each other, but may be able to correspond, even if there are years between responses. There may be practical problems with sending a coherent transmission over multiple light years. If so, instead, unmanned “courier” craft might be exchanged. Such vessels would be one-way only, so are able to expend all their fuel in reaching a high sub-light velocity. They can also utilize propulsion systems and acceleration rates that would have been detrimental to a crew. On reaching their destination system, such couriers would broadcast the data they were carrying. Rather than expending energy decelerating (which might have required weeks or even months) couriers would be programmed to safely fall into the systems’ sun(s). Theoretically a courier could carry physical objects between systems, but this adds the complication of decelerating the “gift package” at its destination.