If he did not, there would be no Kel’Thuzad. Without Kel’Thuzad, there may have been no Legion invasion, although the Dreadlords may have found another Lich to become summoner, idk.
But, let’s run with that. With no Kel’Thuzad and no replacement Lich, the portal wouldn’t have been opened to the Twisting Nether. Would the humans have, eventually, been able to rally together and drive the scourge out of the Eastern Kingdoms? Maybe, but then what?
Assuming they would not have gone back to Northrend, you’d have this weird situation where the Scourge is sitting in Northrend with Mal’Ganis and Tichondrius looking over them. Ner’Zhul is pissed because he now has NO way out of this mess and will be the Legion’s puppet for all-time.
So, Arthas goes back home to give his father a hug (without the stabby-rip-stab-stab), Jaina maybe forgives Arthas? But then what? Medivh now serves no purpose because the Legion isn’t coming, but he already told Thrall to go to Kalimdor.
So, now Thrall & Co. are in Kalimdor without any humans there and there will be no “oracle” because Medivh is like “well, shoot. . . now what?” The orcs probably still piss off the Night Elves, but Grom can’t kill Cenarius without Mannoroth’s blood. Grom and the Warsong clan die for nothing, Thrall goes into Stonetalon Peak and finds. . . nothing. So he probably sets up shop in Orgrimmar, anyway, because well. . . they aren’t going back at this point.
So, what happens then? Does Cenarius come out of the woods and just wipe the Orcs off the face of the map? I have no clue.
EDIT: The big question is – what if Maiev was honest with Furion about Tyrande just being carried down the river instead of being “Torn apart”? Furion would have had them save Tyrande first, giving Illidan time to complete the channeling using the Eye of Sargeras and cracking open the Lich King’s tomb causing him to die and Arthas. . . well idk if Arthas would have died, but he’d have been “taken care of.” Then none of the rest of the storyline would have been done.