Put it this way - animation is costly and time consuming. Cheaper animation looks, er, cheap.
I saw The Invasion, I'm glad they animated the missing parts but seriously, the animation looks like crap. I would prefer a South Park look if it meant more episodes in a short amount of time, that sort of animation is done incredibly fast and I'd be very surprised if it's not much cheaper than what we saw on The Invasion.
I think it's a mistake to try and recreate the original look as closely as possible, it doesn't matter how hard they try, it will always look animated, so embrace the animation part, go for an interesting but affordable style and get the episodes out.