Why is the opposite of 'ascension' still 'descent', and not 'descension'? (Reference. NB: fully aware the answer may be convention, as it is with many things in English.)
Ascension (c.1300) and descension (early 15c.) are both derived transparently from Latin ascensionem and descensionem, nouns of action formed from the past participles of ascendere and descendere.
Descent (c.1300) is filtered through Old French descente “descent, descendance, lineage”; ascent (c.1610) is derived from English ascend by analogy with descent/descend.
The words are close enough in meaning and etymology to mix and match them as synonyms and antonyms for most senses, although ascension has religious senses that ascent does not, and descent has genealogical senses that descension does not. Descension is less often paired with ascension simply because the word isn’t much used in English at all.