While it's technically correct, all of this "AA's only obligation is to get you to PHL" talk is overly alarmist and not how things work in practice.

I've had irrops occur in similar situations multiple times, and not once has AA refused to rebook me via a desired connecting point, if that's what I needed and originally booked. All you need to do is speak up and know your options.

For example...RDU-DFW-ORD-PVG with an overnight in DFW. RDU-DFW cancelled, AA happily rebooked me RDU-CLT-DFW that evening. They didn't "force" me to take the quickest route to PVG, or anything like that.

If the OP really needs to fly PLS-BOS-PHL, it would be downright ridiculous to spend $300 more to book it as separate tickets based on the worry that AA wouldn't get them to BOS for whatever the reason in the event of irrops.