Yes, I'm sure it is scary if you have lead your life assuming that there was a blissful afterlife. But you must recognize that that was Jesus' main selling point: follow me and you'll have eternal life. Judaism believed that when a person dies, they are simply gone. Yet somehow they found purpose and meaning and weren't frightened of a yawning chasm of eternal emptiness.
> when we all die and no humans are left, that'll be it for all of us. At that point,
Yup, just like the universe existed for 14B years before humans came along.
> will anything that transpired previously matter at all?
Here is a thought experiment I've heard before. Say I'm really rich and have a tropical paradise, and I invite you to spend a month there. "It'll be great!" I promise, so you come. When you arrive you find I'm actually a twisted genius. I have developed a pill which will completely reset your memory back exactly like it was 30 days prior. In front of you is a spinner which alternates between: "party" and "torture".
I tell you, go ahead and spin. If it comes up on "party," you'll have the most amazing time for the month. You'll have private snorkling trips with Jacques Cousteau's grandson; you'll eat the finest food cooked by 5 star Michelin chef's I have flown in. Name your favorite bands and I'll arrange for them to fly in and perform for you. No expense will be spared to entertain you. But if you spin "torture", oh boy, it will be bad. Nothing that will leave a physical scar, mind you, thinks like waterboarding, being shocked by high voltages, capsacin injected into your urethra, etc. But at the end of 30 days, I'll give you a pill that will wipe any trace of memory from the previous 30 days, no PTSD or future nightmares.
So the question is: does it matter if you spin "party" or "torture"? After all, in 30 days, there will be no memories nor after effects. I'm sure that just about everyone would pick party. Why does the choice matter? Even though the memory of the torture will be wiped out, while you are actually experiencing it it is very real suffering (or joy in the other case).
To someone who doesn't believe in the afterlife, the fact that all of our memories dissolve when we die (in the case of dementia, even before we die) and nothing outlives our body, the joy and suffering we experience in life matter.
> ... sin ...
Look up the definition of sin: transgression of God's moral law. As someone who doesn't believe in God, I also don't believe in sin. I believe some events are good and some are bad, but I don't believe in sin. I am consciously or unconsciously worried about upsetting God by transgression His laws to the exact same degree you are consciously or unconsciously worried about upsetting Odin for transgressing his wishes.
I said originally I don't want to argue theology. I was just pointing out that non-believers are not nihilists, contrary to the claim of the person I was responding to. I have lots of thoughts about the other things you've written, but this doesn't seem like the right forum for them.