People are pessimistic precisely because they are aware that if we don't do something about the climate crisis, everything will come crashing down. Being optimistic about it, you might say "don't worry, they'll work out a plan" (whoever
they are), and then carry on as before, but being pessimistic, you're more likely to think "we'd better do something, and NOW!".
When you realise that Live Aid was one of the earliest reactions to the climate disaster, and that it was OVER 30 YEARS AGO and no real progress has been made: CO2 emissions have continued to rise exponentially, almost unabated.
When you realise that, you realise there is no cause for optimism.