BA actually fly 2x777 daily to PHL.
US have 1x333 daily.
AA/US have offered to provide a slot for a competitor to fly the route on a daily basis.
This doesn't necessarily mean the combined AA/BA/US will reduce the daily frequencies ,as the competitor slot could be sourced from elsewhere.
It could be very tough for any competitor taking up this slot as both LHR and PHL will be Oneworld hubs , and a once a day service against 3 Oneworld services could struggle.