Do you think that the Others deliberately times their return at the worst possible moment? I feel that even without dragons a united Westeros would have been able to deal with the Others. Valyria before its doom had 300 dragons the sic on the Rhoynar, and presumably more on the homeland, before they all died. The Free Cities could have also provided help, and I think that they were built after the first Long Night. It feels that petty infighting is what makes the coming winter so dire.