Well, presumably there will be less total connecting traffic carried, redistributing it to other hubs. They also mention using larger planes on PHL-BOS, but there are already lots of A321s and 757s (and A330s in the winter), so it's not clear to me how much bigger they can go.

For local traffic, flying PHL-BOS only saves about two hours (door-to-door) relative to the Amtrak Regional for us, and we have no trouble getting very cheap fares on AA. The Acela cuts that difference by 45-65 minutes. So I have trouble imagining that even the local traffic is hugely lucrative. Or at least the advance-purchase local traffic, which they can revenue manage away and leave plenty of space for the short-notice local traffic at high fares.