"were on the mend" can be replaced with antonyms such as "deteriorating", "failing", "worsening", "regressing", or "declining". These words reflect a negative progression or state of health, rather than an improvement. It is important to understand the context in which these antonyms are used, as they may vary based on the situation. For instance, the state of an illness, economy, or relationship may be described using these antonyms, but the opposite of "were on the mend" may be "stable", "steady", "improving", "flourishing", or "thriving". Vocabulary expansion can lead to clearer and more precise communication in both written and spoken forms.