Frankly, thank goodness.
After everything else they’ve done in Shadowlands to utterly demoralise me regarding my favourite fantasy universe, I think it would have been devastating for them to ruin Arthas too.
Edit: Look, Arthas could have been cool. In one way I hope they resolve him, especially with all of the Jailer’s supposed links to Arthas. But at the same time I genuinely don’t have faith that it wouldn’t be done in a cringey way that everyone hates. Like “I will never serve”