Antonyms for the phrase "in a decline" can include "in an ascent," "on an upswing," "improving," "advancing," "thriving," "booming," and "flourishing." These antonyms suggest a positive trajectory or growth, as opposed to a decline. For example, "on an upswing" refers to an upward trend, while "thriving" suggests a state of health and prosperity. Using antonyms for "in a decline" can add variety and nuance to one's writing, highlighting the ups and downs of various phenomena. They can also be used to inspire hope and optimism, suggesting that even when things seem bleak, there is always the possibility of improvement.