I seem to remember that early in the EU's 'review process' for the UA/US merger that they raised concerns about 'total *A presence' in certain airports, specifically Frankfurt. In fact, it said it would suggest the combined UA/US give up some slots at FRA to even things out.

I wonder whether they'd have a similar take if US simply joined *A rather than merging with UA? That would be bad news for me (PIT based with nonstops to FRA).