If it is too late to do anything, why should we care? We can’t reverse it, so why should we care about slow down?
We must limit the problem, then adapt and mitigate. Some damage is irreversible, it does not mean that it’s a good idea to stop trying to understand what will happen. You don’t stop weather forecasts when a hurricane touches land just because it’s going to happen anyway.
Reality is not binary. There’s a whole spectrum of situations between "everything gets back to normal and all is well" (which was never on the cards after the 1980s) and "all humans die within a century". And the nuances in between still affect billions of people.
This is like punching a hole in your wall and saying 'there's already a hole, why shouldn't I just demolish my entire house'