… Deathwing had to be retconned back to life after the Alliance alone ground him into gibbets, and even that was just a side quest. Other than his literal plot armor, he only got as far as he did by kicking the planet in the nuts when no one was looking.
Take that away and he wasn’t even a match for Alexstrasza.
As for what Thrall said… yeah, he also said Garrosh was a good candidate for WarChief. How’d that turn out again? Despite being a “Farseer” Thrall’s uniquely unqualified to make these kind of decisions.
Deathwing was a minion. A secondary threat. A tool. A pawn. A fool, who, we know thanks to the magic of time travel, turns out could be defeated with a really good impaling. He was yet another evil dragon, inflated to carry an expansion which was largely regarded as WoW’s weakest until WoD came out.
Arthas, meanwhile, punked his bosses, out-maneuvered everything thrown at him from his own traitorous minions to Illidan, took control of the Scourge, and took to world conquest with the casual disregard of playing a game. Oh, and as for the powers that made Deathwing relevant? “Hey look! Free building materials.”
End of the day, whatever had to happen to subvert Deathwing’s Old God juice, happened legitimately, happened because of the efforts of those working against him, and happened because he couldn’t stop it despite his best efforts. He fought, he lost. Arthas?
Arthas had to be Deus Ex Machina’ed to death. No other major force we’ve had to content with can say that. The paragons of the Legion, the Old Gods and their minions… Nothing. You can beat Deathwing and the Avatars of the Titans and the Old Gods. Arthas? Only Arthas’ own arrogance and the literal most powerful force in existence can beat Arthas.
The Lich King was the results of the necrotic Manhattan Project of the immortal interplanetary doomsday army who eats worlds for breakfast. Deathwing was the brainwashed minion of a handful of failed prison inmates. The narrative and concepts at play are very clear.
Arthas was the superior villain in every way.