Well BWI competes with IAD to the south and PHL to the north on the USA-LHR route where as PIT doesn’t have any real competition for a couple hundred miles. If the numbers stay strong this year I could see BA switching the PIT flight time around to gradually build towards daily service. But daily service depends on slot availablity at LHR going forward till the new runway is open.
The only thing that might help BWI in this case would be their catchment area, which they claim includes parts of the Philadelphia metro area. But, it sounds as though PIT's location relative to other TATL service (or lack thereof, seeing as the closest TATL airports outside of PIT are CVG and BWI/IAD) could be an advantage for BA if they do eventually go to daily service from PIT.
2019: DAL, MCI, PHX, LAS, DFW, SAT, ORD, SLC, SEA, DTW, PHL, MIA, LAX; B73G (WN x3), B738 (WN, AA, DL), A20N (NK), MD83 (AA), B788 (AA x2), CS1 (DL), B739 (DL), B712 (DL), B752 (AA), B763 (AA), B77W (AA), B789 (AA)