First, there is nothing speculative about death. We all die with 100% certainty.
Second, speculation about suffering after is not pointless at all.
It is possible to reason about it as follows: * First possibility: with the physical death of Aloka-D, Aloka-D's suffering is extinguished. * Second possibility: with the physical death of Aloka-D, Aloka-D's suffering is not extinguished.
According to classical logic, and the law of the excluded middle, these are the two possibilities.
If you accept or tend to believe the first possibility,
If you accept or tend to believe the first possibility, then the object of the dhamma, the cessation of dukkha, is somewhat pointless. Your suffering is automatically extinguished at death, so you don't need to worry. Ultimately, the problem of dukkha will solve itself. All you need to worry about is avoiding suffering as good as you can until you die.
This can be achieved by a number of different methods, one of which is dharma lite.
If you accept or tend to believe the second possibility, then the object of the dhamma, cessation of dukkha, becomes an existential necessity.
Even if you enjoy perfect health, wealth, family, friends and your life is one big party, you cannot escape dukkha.
In this case, there is not only an urgent requirement to address the cause of suffering in the here and now
but it there is also an urgent requirement to cultivate and develop the precepts and the eightfold path in order to prevent future unfortunate rebirth. Now which version did the Buddha teach?
The sutta support for the idea of continuously cultivating the path over several lifetimes flows naturally from the idea of stream-entry, once-returner, none-returner. So you can draw on any of the suttas where these appear, inlcuding MN 117.
As to the other points: nirvana/nibbana is not guaranteed, as Buddhanature exists, but is not necessarily actualised. N
Yes, the ultimate goal is the end of rebirth which coincides with nirvana.
A world full of compassion would certainly be nice, but the Buddha never declares such a goal.
He does not talk about changing the world, but he talks a lot about liberation from the world.
Finally, how rebirth is relevant to practice is explained in the paragraph above addressed to Aloka-D.