We show that asymptotic (valued differential) fields have unique maximal immediate
extensions. Connecting this to differential-henselianity, we prove that any
differential-henselian asymptotic field is differential-algebraically maximal, removing
the assumption of monotonicity from a theorem of Aschenbrenner, van den Dries,
and van der Hoeven. Finally, we use this result to show the existence and uniqueness
of differential-henselizations of asymptotic fields.