IAD: Perhaps it falls between 2 stools in that it's not really a P2P destination with sufficient demand but neither is it a major transfer hub. I imagine the PHL flights have a high proportion of connecting pax (unless someone has specific figures) which would probably impact on an IAD service.
TCX: I see Seattle is now included again on the TCX S2019 timetable which is reassuring. No BOS still, but there are 40 x weekly long haul departures compared to 44 in s2019, but up from the 38 previously showing.
Whether the program has now been finalised, who knows.