If
Sauron regained
the Ring, he would become so powerful that
Gandalf himself said that he could see no way he could be overcome (in
Middle Earth, of course, not from
Valinor). This is one of the reasons for the debate at the Red Horn Gate when
Aragorn tells Gandalf that he does not really approve of the present plan (one assumes to take the Ring to the Fire). Perhaps Aragorn wished to keep and hide the Ring until all the
Elves were able to leave Middle Earth (thus protecting his "family" -
Elrond and his sons and, of course,
Arwen). After that, he might have considered trying to destroy the Ring with the help of Gandalf, but I really believe that he was not sanguine about the chances of the Fellowship actually succeeding in the Quest at that point in time.
If the Quest failed, there would not have been sufficient time for the Elves to flee (with the exception of those already at the Havens) and all of Middle Earth - including
Rivendell and
Lothlorien (as well as
the Shire, of course), would have been engulfed in a new Dark Age. What happened to those taken alive by Sauron's forces does not bear thinking about and that is why Aragorn was so concerned for Arwen and Elrond.
The only hope for Middle Earth would have been assistance from Valinor as had happened once before against
Morgoth. But certainly, no power in Middle Earth could have stood against Sauron once he possessed the Ring.