Simple reason that there is nothing else around to replace them immediately , but they will be gone in around 5 years bar the ex GW HST sets having a new life in Scotland. The one bad mistake BR did on the HST's was perpetuating the daft slam doors - drop window , lean out at an angle of 45 degrees , lean down to open door via handle. Back in the days orf your - almost everyone knew how to handle a slam door - now it is really quite exceptional and people get phased with this quaint railway custom. Similar coaches built for Ireland had customer friendly "plug doors" -(with buttons) - I blame the Derby design crews who were excellent boffins but not really train users ..