Games with non-real-world rules are how everyone who fights wars prepares for wars.

The confusion here is in the word "cost". In the context of lsusr's post, costs and cheapness are framed in terms of monetary costs and cheapness, yet I ask: why not consider moral costs as real, decision-critical costs? Then seek to reduce all decision-critical costs, whether moral, instrumental, or otherwise.

Whether an act was a war crime is independent [LW · GW] of whether an act was the correct choice. Shooting a civilian is murder, whether or not the action is correct. 

it sucked less than the other options that were available at the time.

What if they had dropped nukes near cities—close enough to scare, far enough to not cause serious civilian casualties? Reading more, there were reasons bombing eg Tokyo Bay wouldn't have worked. So you may be right.

I'm under the impression that bombing cities wasn't even effective for that goal. E.g. this discussion:

The case for strategic bombing against industrial targets is marginally better [than the case for terror bombing civilians], but only marginally. While airpower advocates, particularly in the United States promised throughout WWII that bombing campaigns against German industry could lead to the collapse of the German war machine, in the end many historians posit that the real achievement of the campaign was to lure the Luftwaffe into the air where it could be destroyed, thus denying the German army of air cover and close air support, particularly on the Eastern Front. Some dimunition of German industrial capabilities was accomplished (though it is not clear that this ever approached the vast resources poured into producing the large numbers of extremely expensive bombers used to do it, though the allies had such an industrial advantage over Germany, forcing the Germans to fight in expensive ways in the sky was a winning trade anyway), but the collapse of German industry never happened. As Richard Overy notes, German industrial output continued to rise during strategic bombing and only began to fall as a result of the loss of territory on the ground. Needless to say, ‘strategic bombing can sucker the enemy into wasting their close air support’ was not the result that airpower advocates had promised, nor could it have broken the stalemate.

I don’t want to oversimplify the continued debate over the efficacy of strategic airpower here too much so let’s just say that the jury is still very much out as to if strategic airpower works even with modern technology

I don't have a strong object-level opinion, but I note that it's possible that some of the bombing of cities in WW2 was (or should be considered) a war crime, and some wasn't (or shouldn't be). It might be more helpful to think about specific (possibly hypothetical) bombing of specific (possibly hypothetical) cities, than about the general category "bombing of cities in WW2".