go down vs decrease

go down is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
go down	No	No	Yes	No
decrease	Yes	No	Yes	No

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

Other hypernyms of go down include diminish, fall, lessen.

go down (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

go down (verb)	decrease (verb)
grow smaller	decrease in size, extent, or range
stop operating	make become smaller
be defeated
be ingested
go under
disappear beyond the horizon
move downward and lower, but not necessarily all the way
be recorded or remembered