declension vs decline in quality

declension and decline in quality both are nouns.

Word	Noun	Adjective	Verb	Adverb
declension	Yes	No	No	No
decline in quality	Yes	No	No	No

As nouns, declension and decline in quality are synonyms defined as:

declension and decline in quality: process of changing to an inferior state

Other synonyms of declension include deterioration, worsening.

declension (noun)	decline in quality (noun)
a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms	process of changing to an inferior state
a downward slope or bend
process of changing to an inferior state
the inflection of nouns and pronouns and adjectives in Indo-European languages