They could have rerouted on other airlines sooner as well. I would not go down in such argument that BOS was open or not. BA should have send passengers on time, BA could have a spare plane, could have a spare crew. As others said it's BA who has to explain why is it extraordinary and out of their control and not you need to prove this is not extraordinary. I would only ask a question from myself that if the plane leaves on time would be BOS open? Any other airline managed to land BOS or not? Let say there was a massive storm which closed an airport and tens of flights needed to divert then yes you won't have much argument over this. However I haven't seen in their response other than if they send a scheduled flight many hours late then the arrival airport would be closed or not able to accept your flight because of lack of slot/gate/etc. That's not the problem here the problem is what BA has to explain why your plane haven't left the gate on time. Their answer just say some sort of resourcing issue which is not extraordinary at all.