As
@Composer99 points out, the issue with that is that you cannot alter the fighter class, if you must keep the Champion as is, with few to no options. Any complexity you build into the fighter, cannot be avoided by the champion.
Take an example of the new Rogue. The Cunning Strikes feature is amazing! And every subclass plays with that feature, adding new layers to it, and new interpretations to it. You cannot have a rogue subclass though that removes the cunning strikes from the base class. That doesn't make sense.
If we wish to rewrite the fighter, but not change the champion, then shifting our expectations to the Warrior Class which is not the fighter, is a clean and elegant way of doing it. Now, you don't need to alter the "simple class" but you can take "the fighter" and make it more complex with interesting abilities.
That would be great, but people like you have stated, vociferously, that any change to the Champion that would add knobs and dials would be a deal-breaker and leave the proposal dead-on arrival.
So, if we instead move that simple class with no knobs and dials to the Warrior, then that allows you and people like you to keep what they want, an already published, simple class, with no complex mechanics, and frees the rest of us up to alter the baseline fighter to match what we desire.