Yes, that is a critical point: the current timetable is based on 60 minute MCT, typically offering 60-120 minute connection on many itineraries, and unless the whole timetable changes, so that connection "banks" or whatever they are called these days are spaced up, which seems hard to imagine given the rigidity of LHR slots, the effect on many itineraries will not be to lengthen them by 15 minutes but often by a lot more as, if a flight existed within 60-70 minute of a given that can no longer be selected, the next available one may well be around 180-220 minute after arrival instead.

BA is already not the most obvious choice for connectors in the European landscape, now some itineraries will look even more significantly longer than AF, KL, LH or LX equivalents, all of which are already advantaged by greater convenience (not least lack of transfer security on many itineraries in both directions).