Defining monopoly by marketshare is shallow. Best monopoly is when your customers switching costs are high. Thats why some of FAMG is with so good friends with Lawmakers. To provide political protection.

I am not from US, but what I see from my experience there is one huge problem with mergers (besides all other): too big to fail. If you become systematical player like big banks and if you intentionally or non-intentionally get in trouble. You will be bailed out, not go bankrupt. This turns on God mode for Corporations, this create intention become reckless. If big banks would not be bailled out in 2008. Most likely we would see at least some kind of fragmetation.