The thing about pessimism is that it's a very attractive mindset for people who don't like to think all that much, but want to have answers.
It's also the mindset for people who stagnate and adopt a defeatist attitude.
It's also an extremist mindset, that closes itself away from other options and is fueled constantly by more pessimistic thoughts.
It's also a mindset that can quickly lead to depression, due to the constantly negative and skewed perspective of life.
It's also an easy mindset to disguise as "realism". There is no realism in what you've advised, in fact the advice for people to stop reproducing due to a doomsday scenario is perhaps the least realistic thought when it comes to reactions to global warming. The absurdity of such an advice is only explained by the naivety of someone who only now started life (which is accurate).