Palliative's primary meaning is to treat or mask symptoms rather than the disease itself, and it has nothing to do with duration. Current antiretroviral therapy for HIV is for life, as far as we know, but this does actually treat the disease itself by controlling the virus. Painkillers for a broken leg are generally only needed until it heals, but they are not causal in that healing, and are properly called palliative.

One cannot make a case that any given treatment for mental illness is merely palliative on the basis of duration.

Lifelong treatments are indeed not curative, but they are better than palliative care, which is better than nothing.