decline vs come down

decline is a noun but come down is not a noun.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
come down	No	No	Yes	No

As verbs, come down is a hyponym of decline; that is, come down is a word with a more specific, narrower meaning than decline:

As verbs, come down is a hypernym of decline; that is, come down is a word with a broader meaning than decline:

Other hypernyms of decline include descend, fall, go down.

decline (noun)	come down (noun)
a downward slope or bend
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	come down (verb)
grow smaller	get sick
grow worse	be the essential element
show unwillingness towards	criticize or reprimand harshly
inflect for number, gender, case, etc.	move downward and lower, but not necessarily all the way
go down	fall from clouds
fall in value
not accept as true