I didn't say "world ending". Only you are referring to the world ending. The term being used by me and others here is "post-apocalyptic" and you seem to be conflating that with only meaning the world literally ending, but the term is not as simplistic as that.

Meaning of post-apocalyptic according to Oxford:

Following a large-scale disaster in which civilization has been destroyed or has regressed to a more primitive level

When I hear "post-apocalyptic" I think of things like society having effectively collapsed or regressed as the result of a nuclear holocaust or some other cataclysmic event and whoever is left is struggling to rebuild with factions fighting over the scraps. The world hasn't ended in this scenario, it has regressed.

Nuclear holocaust and almost pushing human civilisation back to the dark ages while factions fight over what is left is exactly what is described as having happened during the time period following Star Treks WW3.

Judge Dredd, for example, is another universe that is most definitely considered by most to be post-apocalyptic even though Earth still has a massive population living in continent sized mega cities. Judge Dredd is set during a time period where humans are experiencing societal collapse a few decades after a nuclear holocaust. This is similar to how i imagine Earth is in Star Trek immediately following WW3 during the time period known as the Post-Atomic Horror. And we are shown a snippet of that collapsing society by Q during Encounter at Farpoint.

Comparing this kind of scenario with our WW1 or WW2 doesn't work because neither of those wars set back human civilisation. Even considering the devastation and human loss, human civilisation experienced massive growth spurts in technological expansion both during and immediately after those conflicts. Humanity went forward, not backward.

Comparing it to the Black Plague also doesn't work because, while devastating to the population, society did not collapse. The Black Plague did not result in any countries being wiped out or collapsing.

I think this debate is more semantics than anything. Human civilisation in the Star Trek universe has experienced a global apocalyptic event resulting in all of Human civilisation being pushed backwards significantly. This is not the same as a devastating event confined to certain regions like both real WW's (even as massive as they were) and the Black Plague. The shows take place after these apocalyptic events, therefore the shows are post-apocalyptic, but the shows themselves are also set so far beyond that time period they themselves could also be considered post-post-apocalyptic as humanity has recovered by that point.

It's a post-scarcity meritocratic society brimming with life and good values and which posses near godlike technology and interstellar travel.

Exactly, but we also learn from Picard in the first episode of TNG, during the encounter with Q, that humanity has achieved this level of society as a direct result of learning from the horrors of WW3 so that they don't repeat the same mistakes. That post-scarcity society evolved from a post-apocalyptic society. Therefore the show can be considered both post-apocalyptic and as you describe. The two descriptions are intertwined and both valid. Just because the underlying post-apocalyptic side of the shows theme isn't apparent at first glance doesn't mean it isn't there. It serves as the origin story of the post-scarcity society on the display in the show.