How much has Shadowlands fundamently changed Arthas's story?

[TickTickTick]

control of the scourge must be maintained or else ... Sylvanas breaks the helm and nothing runs rampant

There is a way to twist it to make a bit more sense though.

Let's say, Helm of Domination was getting directly powered up by those slain through domination magic, so basically LK and his scourge. The plan was always for the Helm to accumulate enough power that its shattering would break the veil. What the Jailer didn't anticipate was that both Ner'zhul and Arthas would be able to resist his will to some extent and they went rogue. Especially Arthas, resulting in Azerothians assaulting him earlier than planned. The Helm was not powered enough yet and it was pretty obvious it would be destroyed by Tirion.

All of a sudden ghost of Terenas appears and tells him "there must always be a Lich King". We've already seen with Sylvanas and Uther that one part of soul is sent to Shadowlands (both parts in the case of Uther, one to the Jailer's prison and one, supposed to be absorbed by Frostmourne, to Bastion). And we can see with Sylvanas that the piece absorbed by Frostmourne (in her case instantly released to raise her as undead) is the... darker one. Maybe it was the part of Terenas' soul that was easy to dominate by the Jailer, maybe it wasn't his soul at all and instead some of his magic trick. Anyway he said what the Jailer wanted: another Lich King must be crowned (not because the scourge would pose a mortal threat if he wasn't, but because it needed more power).

Then, when everything is ready, when the Arbiter is shut down and the Helm has enough power, he ordered Sylvanas to shatter it.

[VinceVega]

I don't think it changed anything at all. You only got more knwoledge of how stuff happend and stuff we have been told buy unreliable sources has been prooven well.. not quite correct.

It basically just superimposed another layer of story onto what happen to him and what he did. But not why he did anything. His motiv, is way of doing things and what he actually did never changed.

[shadowmatrix]

There is a way to twist it to make a bit more sense though.

Let's say, Helm of Domination was getting directly powered up by those slain through domination magic, so basically LK and his scourge. The plan was always for the Helm to accumulate enough power that its shattering would break the veil. What the Jailer didn't anticipate was that both Ner'zhul and Arthas would be able to resist his will to some extent and they went rogue. Especially Arthas, resulting in Azerothians assaulting him earlier than planned. The Helm was not powered enough yet and it was pretty obvious it would be destroyed by Tirion.

All of a sudden ghost of Terenas appears and tells him "there must always be a Lich King". We've already seen with Sylvanas and Uther that one part of soul is sent to Shadowlands (both parts in the case of Uther, one to the Jailer's prison and one, supposed to be absorbed by Frostmourne, to Bastion). And we can see with Sylvanas that the piece absorbed by Frostmourne (in her case instantly released to raise her as undead) is the... darker one. Maybe it was the part of Terenas' soul that was easy to dominate by the Jailer, maybe it wasn't his soul at all and instead some of his magic trick. Anyway he said what the Jailer wanted: another Lich King must be crowned (not because the scourge would pose a mortal threat if he wasn't, but because it needed more power).

Then, when everything is ready, when the Arbiter is shut down and the Helm has enough power, he ordered Sylvanas to shatter it.

Or we could just say Zovaal wanted the crown to keep being used, sooner or later someone would get the crown who could be controlled. He ordered the crown destroyed so the Mawsworn could directly bring select living beings directly into the Shadowlands while bypassing Oribos.

[Romanthony]

This is the problem with Retroactively, altering all past Lore to make your current Villian seem more "Threatening" you undermine your better Villians, as someone said earlier.

"Arthas, the most recognizable villain in Warcraft is now someone's failed attempt at a minion "

And i still dont understand why they decided to retcon decades of lore, for the jailer, just to kill him off in the same expansion he was introduced.