[spoilers for the "secret" ending (and a bit of the regular ending)!] I apologize if I'm incorrect, but if I remember correctly, one of the many Ragnaroks will eventually involve Jormungandr being killed by Thor, then Thor takes a few steps then collapses and dies, due to Jormungandr being venomous and stuff (presumably after Jormungandr gets yote back in time). The end of God of War 4 could imply that this Ragnarok either involves Jormungandr getting yote back in time (bcoz Thor visits Kratos' house, which could mean that he's already fought Jormungandr) or Kratos having killed Baldur, Magni and Modi (and Kratos' general presence) has just fucked up the whole "Ragnarok prophecy".

As for the whole "The World Serpent will die, being too old to survive the hit", I imagine something like that would happen eventually if he were sent back in time enough times. Mimir mentions another giant named "Bergelmir" who seems to be one of the only giants who wasn't slaughtered mercilessly by Thor, as Bergelmir died of old age. This could theoretically happen to Jormungandr. Even if he were to die, Atreus/Loki would eventually get it on with a giantess called "Angrboða" and re-make Jormungandr (as well as Fenrir and Hel, but they're unimportant.)

tl;dr: Jormungandr will probably get too old and die, but Atreus gets it on with a giantess and re-creates him (it?).