Does it take a blanket presupposition for a historian to discount some miracle stories as legendary? No, because, as even Bultmann recognized, there is no problem accepting reports even of extraordinary things that we can still verify as occurring today, like faith healings and exorcisms. However you may wish to account for them, you can go to certain meetings and see scenes somewhat resembling those in the gospels. So it is by no means a matter of rejecting all miracle stories on principle. Biblical critics are not like the Committee for Scientific Investigation of Claims of the Paranormal.