I think we're getting a little sidetracked here. What the grandparent wanted to say was just that the corset of laws that binds a judge might not be as tight as you think; hence e.g. activist judges.
I think my point stands, which is that not every possibility is an eventuality, and we shouldn't assume that to be true without a willingness to accept a very bad panoply of preventative measures masquerading as corrective measures.
That said, whether or not an activist judge allows a presumptively lawful merger to occur is hardly a hill worth dying on. We have remedies for activist judges through an appeal process. Where an appeals process is not available after the merger, we have remedy for anti-competitive companies through regulatory influence, regulatory agency, rule of law, and a court system to back that up.
