dwindling away vs drop-off

dwindling away and drop-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
dwindling away	Yes	No	No	No
drop-off	Yes	No	No	No

As nouns, drop-off is a hypernym of dwindling away; that is, drop-off is a word with a broader meaning than dwindling away:

Other hypernyms of dwindling away include decrease, lessening.

dwindling away (noun)	drop-off (noun)
a becoming gradually less	a change downward
	a steep high face of rock
	a noticeable deterioration in performance or quality