My personal desire for Minthara is not to change how she can be recruited, but to give the evil route a lot more content to make it truly worthwhile, beyond getting a single companion while losing countless others. Going evil locks the player out of so much content, and Minthara does not feel like enough to make it a truly fun gameplay experience.
I've seen arguments like yours a dozen times since launch, but what most people don't realize is expanding the 'evil' route (which doesn't really exist because the game isn't clearly divided by something so binary as 'good' and 'evil') would require even more effort. There is a reason why Larian isn't adding more to the 'evil' route and has gone on to plan the KO option for Minthara. You're free to hope, of course, but I personally have already given up on that one.
You know, you bring up a lot of good points. Honestly, I agree with you. The KO option is just the easiest to implement, though, since the events of the game wouldn't have to change at all. So, that's probably what we're gonna get. Still, at least we don't have to be evil to get her, which is wonderful because that's something I don't have the heart for. I'd rather be called an idiot by her than let the sweet tieflings die

Because it's the "easiest to implement" that somehow makes it okay and leaves no room for improvement? There is actually a lot that can be done without changing the course of the game, but I understand some people lack the imagination to suggest something worthwhile. I just expected better of the writers at Larian.