I too believe that US carriers are voting with their feet is a load of tosh. The experience isn’t great for sure but, unless there are notable reductions in pax numbers, do you honestly think the airlines care that much? And why do you think that UA minds but LH doesn’t?
Transatlantic has always been business and leisure orientated and both have taken a battering. Going East, the carriers have the added benefit of strong VFR traffic which isn’t as apparent to the states.
DL has VS at MAN, AA’s heart wasn’t particularly in it before covid and that was nothing to do with the airport experience either. That leaves UA.