climate change isn't an one/off effect but gradual

every bit of improvement is a higher chance to avoid some of the most catastrophic outcomes (where the unlikely but possible worst outcome being a mass extinction chain reaction which humanity will find very very hard to survive in a functioning manner/without losing their future)

so still worth fighting for any improvement even if we can't avoid a catastrophe anymore, as there is a huge margin between what we still can archive, and what we might end up with if we stop fighting and are quite unlucky