"They"? You are overgeneralizing a group you are not a member of.
What you're describing covers exactly zero of the "doomers" I know - most are just depressed PhD biologists or polymath economists modeling a tragic collapse.
Look up the revised World3 models, IPCC's BAU scenarios, go review Hot Topic and the critiques since then, check out the Existential Climate Risks survey of experts... talking about the risks of collapse, and fearing it happening, is quite rational.
Casting the group as expecting some absurd "population collapse without civilization collapse", or cheering on a massive societal regression, is a clear signal your sample set is biased.