alternate realities do exisit, but RTDs wasted alternate realities, when he wrote Rose out. The Doctor can not travel to alternate realities now.
Wamdue, when the hell has anyone ever
let continuity get in the way of telling a good Doctor Who
story? If Moffat or the writers and producers that follow want to tell an alternate universe story, do you think that he'll let "Journey's End" stand in his way? If you answer yes, you're daft.