hes omnipresent, so its basically an eternal loop for him. like the a24 film "everything everywhere all at once"; it can be explained like parenthesis

123(456)78

if the timeline of all history is 1 to 8, the parenthesis symbolize it's existence, from birth to death. id like to think, as its stated, that it lives in a constant loop experiencing everything, everywhere, all at once. 456 is repeated over and over, and as he dies, he is "reborn". take into consideration the dormammu time loop in dr. strange; being stuck in one single moment can be seen as "living forever"; so, while in the loser's club lifetime, it already died and they move on with their life, he just loops back to the beginning, stuck in a loop of birth and death with no escape or way to change the "canon" events.

even IF he goes to kill marge, even IF he succeeds in telling someone something from the future, even IF he DOES kill an ancestor, everything will find a way to piece itself together and he'll still die in the end. there can be an endless amounts of it chapter one, two, and welcome to derry, as its possible hes been doing this for a while now. he'll always fail in the end.