If the Ones died, doesn't that mean there's a way Abeloth might die?
Correct. The Mortis Dagger is the only item that might kill Abeloth. The Mortis Dagger is what killed the Ones, and Abeloth is similar to the Ones. But she isn't a One, never was. So it's possible the Mortis Dagger may only wound her. Which is still better than nothing. It would behoove the Jedi to research what the Mortis Dagger is made of. Perhaps they could then craft their own weapons to hurt Abeloth permanently.