Find your tribe in a Sea of Creativity
OK so
Wilbur/Revivebur said he was alone for 9 years or whatever, which means there was no Schlatt, no MD, and no Tommy. But we saw Tommy talking to ask three of them when he died, so how can this be true? Well: personal afterlives.
If each afterlife is personal then they have to be constructed somehow, right? So what if they're made using expectations, hopes and fears. Personal afterlives mean all of the afterlife options can be true:
Revivebur was alone because he hates everyone and himself so he wants to be alone and thinks he deserves it. At first a relief it quickly became torture, but it had been established in his mind already that the afterlife is unchanging. His own personal hell, designed by himself, though subconsciously.
Tommy, having been told the afterlife is a big void by Ghostbur (who experienced Revivebur's afterlife until he split off as a ghost), gets exactly what he expects a big void holding all the dead people he knows. All of them act as he expects them to, Schlatt is drunk/asleep, MD is being a creature of chaos coming and going as he always does, and Wilbur is even more unstable than he was in Pogtopia (which gives me hope for a possible redemption arc, not much though as Tommy knows Wilbur better than anyone, and Revivebur probably has a "dream my beloved" locket)
Everyone knows Ghostbur's afterlife is happy and peaceful, just because Ghostbur and Revivebur rode the same train doesn't mean he got off on the same station. Ghostbur is in a flower field and all his friends are there and he's living his best death. Sometimes some friends aren't there and sometimes they are all different ages. Like Fundy might be a baby one day and an adult the next, Tommy is a toddler and then he's sixteen and then he's ten.
but they are always happy
Sometimes the real Mumza pops into her son's afterlife to check on him. Because she's death and she can enter any afterlife, but only if the person wants her there, which Revivebur Did Not (going off the assumption that Ghostbur + Revivebur = Wilbur/Alivebur).
Why doesn't she visit Tommy? Well, why would she? She doesn't know him, he doesn't know her. The only thing they have in common is that they view Wilbur as family.
Kinda implies Mumza was the train but we're not going to think about that too hard