This assertion is rather short-sighted and presumes, wrongly, that knowledge on a topic is already complete.
Yet, we live in a day and age when scientists are still discovering muscles in the jaw. We're talking about basic anatomy, not how neurons work.
https://news.ycombinator.com/item?id=29705819
Is it too hard to imagine that knowledge on how the brain actually works is still far from complete?
The equivalent of "it's never aliens".
There might be an afterlife or not. But in order to accept this idea as true, a proof (extraordinary one in that case) is needed. And for now, except some anecdata, there isn't much to it (as far as I know).
The basic principle is falsiability. It has been widely established that a sufficient condition to alter or eliminate consciousness and even personality traits is inflicting precise physical changes to the brain, such as severing pathways.
There is absolutely no evidence, let alone reproducible observations, supporting crackpot theories such as afterlifes or any form of consciousness that is not dictated by the brain.
It makes absolutely no sense to appeal to this or any other form of crackpot theories by alleging that established facts and knowledge covered by mainstream theories is still under development.
To assert that atomic activity dictates the bounds of consciousness sounds a lot more like asserting the moon is made of cheese than entertaining the possibility that it isn’t. We didn’t even understand basic atomic principles until Rutherford et al around 100 years ago, and our knowledge regarding neutrinos and particle acceleration is still rapidly evolving.
To claim that because we have not directly witnessed or extra-physical consciousness is a bit like saying a grain of sand is the smallest unit because we haven’t witnessed anything smaller.
The statement that there is no afterlife is equivalent to the statement that the sun is not made of spaghetti. There's no possible math, context, perspective, or arrangement of facts that would allow the sun to be made of spaghetti.
Your consciousness resides in your neocortex, the thin sheet of neurons on the surface of your brain. We know this because when injury or illness disrupts or damages its operation, your consciousness goes away. It's maintained if there's injury or illness anywhere else.
There's no room for disembodied mystery. There's no room in the laws of physics for information transfer such that the pattern of physical connections and electrochemical activity in your brain to be communicated intact when you die. Everything we know about the universe leaves no mechanism for your mind to continue after biological death.
We will eventually scan brains, and at some point we might be able to extend our minds into machines and maintain our consciousness after the biological parts die. We don't know enough about the nuanced particulars of neuroscience but the challenges are a matter of resolution, scale, signal strength, and not damaging a living system. There's no mystery to the phenomena of consciousness allowed by laws of physics.
Multiverse or simulation theory leaves room for some different level of reality where the substrate of your mind resides. Those are interesting abstractions, but there's no evidence or theory to suggest either of those contexts are valid.
It's not arrogant to assert that the moon isn't made of cheese or the sun is not made of pasta, or that there is no life after death.
It seems to me that insisting on those things in the face of evidence would be arrogant, as you would be claiming that you know better than everything that can be proven, all of those things for which we have evidence and explanation.