I disagree. The OG Overmind would have most likely tried to consume and assimilate Amon and his hybrids. It was basically more or less an alien eldritch horror version of Skynet. It turned against its creators, but it still continued with the directive it was programmed. This is why Amon corrupting the Overmind was so painfully unnecessary. If you take that little tumor away, you'd still have something presentable if not good: The Overmind's sole reason to create Kerrigan was for her, due to her massive psionic potential, to become the Queen of the Swarm should something happen to him and the cerebrates, especially when Zasz was permakilled. Her, the greatest genocidal mass murderer in recorded human history, being instrumental to saving the universe from Amon was according to Bob Ross a happy accident.
But I fully agree with what they did to Mengsk. He almost became like Scar from the Lion King, as in he exchanged a functioning brain for the crown.
I actually share your friend's sentiment to a degree. The problem with Starcraft 2's writing is that you have character inconsistencies that completely fuck up the story and rob it of its amazing potential. To bring up Heart of the Swarm again, the ideas and direction on their own were actually good, such as Kerrigan starting to feel genuine guilt, asking herself if it was all worth it and most ironically expressing her human side through her interactions with other zerg characters. God help me, I even unironically loved the mission where Kerrigan and Duran had the Dragon Ball Z laser beam struggle like a reenactment of the first Goku vs. Vegeta fight. It was so exceptional it was awesome. Starcraft meets Dragon Ball Z, it was like I was 16 again.
If I had to rewrite the entirety of Starcraft 2, I'd only make some minor changes. But as you can see, even minor changes can be very impactful.
And finally, the one good thing to come out of the Warcraft III remake's disaster are the memes.