I think we know far more than many people seem willing to give credit for.
We know that damage to isolated parts of the brain will selectively destroy memories, language skills, motor-function, general cognitive ability etc. In other words there is every evidence that who we are is an emergent product of the interconnected workings of component parts of the brain. If we excise or damage part of the brain through trauma or oxygen depletion etc, then part of what makes us who we are, goes too. If we allow the entire brain to decompose, then everything we have learned over centuries of examination and experimentation points to the same conclusion. We cease to exist.
So there actually is a concrete answer to this question. It just doesn’t have a great deal of appeal for those of us who think they’ve somehow been short changed and deserve more than “just” the decades of life we might have a chance to live.
So yes we can speculate. But all of that speculation is in a different camp of probability than the events I describe above. Its only because of cultural/religous/personal investment in the relative merits of one particular unfounded speculation over another that these discussions seem to take on any semblance of reasonableness.
But if we accept that then, granted, we can have completely speculative discussions as to whether one person’s highly improbable and completely unfounded belief is more or less valid than another’s highly improbable and completely unfounded belief.