“Existential threat” means a threat that threatens the existence of something (civilization, humanity, etc). It doesn’t just mean “really bad.” If you have a different meaning, you’re using the term wrong.

And there is nothing “bad faith” about the argument. An “existential threat” warrants a different response than lesser threats. So establishing whether or not climate change is really an “existential threat” is really important to the debate.

Climate change isn’t “existential” even for Bangladesh. A third of the Netherlands is already under sea level. Technology compensates. Experts project that climate change could render 17% of Bangladesh under sea level in 40 years. 90% of Rotterdam is under sea level. The city was originally built using technology substantially more primitive than what Bangladesh might have in 40 years (when it is projected to be a middle-income country with a $3 trillion economy).

Most proposed anti-climate-change policies also threaten “life as we know it”. That’s literally the reason why e.g. the French were massively protesting gas tax hikes.