It's already happened. Facebook, Twitter, Youtube already remove and delete comments that are outside the overton window. Sure, they're not part of the government, but when it comes to freedom of expression does it really matter?
I see your point. But those platforms (while flawed) are operating legally, under their own volition, without direction from the United States Government.
If the USG were to order Twitter to take down X person's tweets because it hurt someone's feelers, that would be a clear violation of the first amendment.
Additionally we have a strong legal precedent of not permitting "compelled speech". Plainly put, they can't force you to label your own Facebook posts with your opponents claims as well.
I am totally on your side that these few companies are in far too much control. But they aren't the USG, or compelled to block/force-speech on their behalf.
