But an actual massive world war looking anything like WW2 just isn't going to ever happen again.
If the situation in Ukraine is any guide, WWIII will look a lot more like WWI than like WWII.
I am not sure why anyone would expect something similar to WWII, and indeed, I am not sure that anyone even
does - saying "anything like WW2 just isn't going to ever happen again" is a strawman, built to make the speaker appear wise while he spouts the bleeding bloody obvious.
Who is saying that WWIII will look like WWII? I am not aware of any serious commentator who says that.
During the Cold War, the near universal assumption was that WWIII would entail a massive thermonuclear exchange. Today that's seeming less likely, (though far from impossible, at least as an endgame to a more conventional conflict).
The two world wars so far have been rather different in character, both militarily and (to a lesser extent) geopolitically, despite both having had Germany star as the lead villain.
It would be strange to expect the next world war to be a near duplicate of either of the others.
The first war led to technologies that gave the second a totally different character; The second did the same for the Cold War, to the extent that it never even entailed mass casualty conflict between the major belligerents. The next might reasonably be considered WWIV, rather than WWIII, but whatever it is called, it's clear that the technologies and strategies from the preceeding wars will ensure that it is very different again.