Some herbaceous perennials have relatively short lifespans but are still well worth growing.
When a perennial starts to show signs of decline, you have various options. You could remove it, taking the opportunity to refresh your display with something different. You could reinvigorate the plant by lifting and dividing, as the decline could be the result of congestion. Or, you could take cuttings or sow new plants from collected seed (although many perennials will self seed, saving you the trouble).