Why wouldn't you consider poignancy to be the opposite of nostalgia?
Poignancy and nostalgia can be related concepts, but they are not necessarily opposite in meaning. Poignancy refers to a feeling of sorrow or emotional pain, often accompanied by a bittersweet or touching quality. Nostalgia, on the other hand, refers to a longing or yearning for the past, usually for experiences, people, or places from one's childhood or earlier times.

While sentiments like poignancy and sadness can sometimes accompany nostalgic feelings, the two concepts are not exact opposites. It's possible for someone to feel both bittersweet nostalgia and the poignancy of lost experiences or loved ones at the same time, for example.