BA have only recently added a 7th flight on the NCL-LHR service so it is highly unlikely that the route would be dropped. Comparing the journey time with the train is not always relevant; if you are travelling point-to-point ie a day's business in London then the train is probably as quick for central London. But I suspect that >50% of passengers are connecting at LHR onto other flights hence BA gets revenue on 2 routes. Drop the route and what happens? You gain a few slots but lose alot of passengers who would move to the trains or KLM/AF feeder flights as an alternative.
I suspect that connecting passenger numbers will also increase on the BA service when LHR T5 opens; as the single terminal concept at AMS(much easier for connections) will have no advantage over BA's new single terminal operation.