Think about it, Denji lives his life a second time without Pochita, gets to actually learn something and put it to practice in a world where all the horrible things Pochita erased before are all present.

Then Pochita gets to come back with some incredible twist like Denji reinventing the chainsaw. Then part 4 would be the duo going through what would essentially be part 1 again but with enough knowledge to avoid all the tragedy and earn a happy conclusion with all their friends alive.

But here's the kicker, Fujimoto probably already knows that I am making this theory right now, his whole writing style is "now what's the best way to destroy the reader's expectations". Which makes him actually ending the whole series in the next chapter all that more likely.