Hi, Edhel!
My guess is that the Prophecy shouldn't have been allowed to stand. Or at least few important elements of it: I don't really think that Tolkien would've kept the idea of making Túrin imortal -- and in the Prophecy he kills Morgoth with Gurthang, which was, however, destroyed and lost in the latest versions of the Tale of the CoH. Moreover, we know from the LotR that Sauron did
not recover the finger that was cut by Isildur. If Morgoth had his legs (and arms too?) cut, he'd not be able to fight, once he was in that same condition of Sauron (and also of "his greater agents", as told in Ósanwë-kenta). That's what I think.