I believe that after our human race becomes extinct, due to either overpopulation, climate change, an alien invasion, a black hole or the ageing sun devouring the Earth, we will vanish into eternity and nothing of our great feats will be remembered or ever preserved. It is quite a morbid thought yes, but sadly it is the truth. There is no after life as after death, we will experience what it was like before we were born; nothingness. Being born into another life in another galaxy or universe is a pleasant but thought provoking situation as we will most likely not have any remembrance of our previous life. This raises another question. Did we live in another universe or galaxy before we were put onto this planet?
I think it is impossible the whole human race will just sink into nothingness. I really can't even imagine being in a state forever like the one we were in before birth. Something else has to happen, though it's not like we'll ever know anyways. I don't think there is any way you can prove we'll just sink into nothingness, nor is there any real way to prove religion.
Your next question is thought provoking, but can answer itself. If we transported here from another galaxy/universe, obviously no one has any remembrance of it, so the cycle could definitely repeat itself. Either fate seems dark, considering you are either sucked into nothingness or forget everything. Nonetheless, the fate of Earth is inevitable but the doomed destruction is not likely to happen anytime soon. Whatever the case, considering how large space is, I think it is completely and utterly impossible that there isn't another planet suitable for human life.
It is possible for the human race to sink into nothingness as scientists have predicted that in 4-5 billion years or so, the sun will explode and ultimately destroy the Earth. That or any other circumstance can wipe out the human race for our galaxy. It is wishful to think that something else will happen but our scientific advances have not proceeded far enough to determine what will happen after the human race goes extinct. Sadly, we may never know.
Is this 'life' just an eternal cycle of us going from galaxy to galaxy or universe to universe? It is almost impossible to answer as we have no evidence but for our sake, let's hope we have another chance to experience functioning as life matter again and not drifting from death into the state we were in before we were born.
Of course there is another planet for human life or just life form, scientists have discovered a planet that they have called Kepler-186 f that is similar to Earth but we do not know if any life form inhabits it but I think for a lot of people, we can assume we are't the only living creatures in this universe. The main point here is that from what we know so far we have only lived on this planet in this galaxy and therefore if you, or I, or for arguments sake the human race becomes ceases to exist, we will vanish into nothingness and we may never even know if we will be reborn into another galaxy or universe. We do not have any internal or external spiritual life forms so as human beings that are made out of bone, muscle and skin, we will not know if we are reborn. We are plainly and simply just animals.
Many scientists today are trying to look for planets suitable for life, like Kepler-186. Since space is so vast, it is entirely possible that another planet somewhere out there has life, though I doubt we'll ever know. As for the afterlife, nobody can confirm what happens to us when we die unfortunately, and that will always be a mystery even if you do believe in a religion. Whatever the case, life on Earth could really end at any time, from a giant meteorite or another strange phenomenon. Seems like all the questions we want answers to will probably never be solved.
For many people that follow a religion, they do know where they are going. Many Christians believe if they are 'good' they will enter Heaven but if you are 'bad' you will go down to Hell. There is a possibility that Earth could end at any time but with our advanced technology today we can predict when or how it will end. Right now, overpopulation is a major issue. I do agree with you that we may never find out the answer what will happen after we die but for many people that have common sense, we can assume that it is the experience we enduring before we were born thus proving that we may sink into nothingness.