That was the case when Standard Oil was broken up and it's not a problem. The retail arm becomes free to get their cloud computing needs from others -- say Azure or Google. Bezos can't force them to get a worse deal at AWS simply because he owns the AWS. A lot of the arguments for breaking up companies come from this. It can make the components more efficient once they're freed from the shackles of being locked in. Likewise, the crappy parts of the big conglomerate can't coast on having a guaranteed customer. This is the whole point of capitalism. Monopolies are the anti-thesis of capitalism.
I don't think it's fair to say the break-up was a failure even if in the near term they still coordinated. In the long term, the oil companies competed with each other. Even if Rockefeller became wealthier, it only adds to the original point: breaking up monopolies unlock value for shareholders and increases efficiency. I highly doubt anyone in the main stream schools of economics would call the Standard Oil breakup a failure. That Rockefeller wasn't punished is immaterial to the breakup itself. The point here isn't to punish Bezos or anyone else. It's about efficiency and fairness. In that sense, that goal was accomplished. No single oil company today wields the power that SO once did.
Also, those guys meeting up together at Rockefeller's house to coordinate would likely be illegal today and probably was back then too.
