The destruction of two oppressive empires which were engaging in largescale genocide would be the most obvious success criterion.

Yeah, but were these oppressive empires really engaging in largescale genocide before WWII or was it (partially) caused by WWII? If the latter, then that isn't a point in its favor. If I remember correctly, before WWII the "final solution" was supposed to be the Madagascar Plan, not The Holocaust. The only oppressive empire I can think of that was engaging in largescale genocide pre-WWII survived the war (and even expanded its power as a result of it).

A more cynical point is that for the US at least it really did help the economy.

This is definitely not a consensus amongst economists. For instance:

It is commonly argued that World War II provided the stimulus that brought the American economy out of the Great Depression. The number of unemployed workers declined by 7,050,000 between 1940 and 1943, but the number in military service rose by 8,590,000. The reduction in unemployment can be explained by the draft, not by the economic recovery. The rise in real GNP presents similar problems. Most estimates show declines in real consumption spending, which means that consumers were worse off during the war. Business investment fell during the war. Government spending on the war effort exceeded the expansion in real GNP. These figures are suspect, however, because we know that government estimates of the value of munitions spending, to name one major area, were increasingly exaggerated as the war progressed. In fact, the extensive price controls, rationing, and government control of production render data on GNP, consumption, investment, and the price level less meaningful. How can we establish a consistent price index when government mandates eliminated the production of most consumer durable goods? What does the price of, say, gasoline mean when it is arbitrarily held at a low level and gasoline purchases are rationed to address the shortage created by the price controls? What does the price of new tires mean when no new tires are produced for consumers? For consumers, the recovery came with the war’s end, when they could again buy products that were unavailable during the war and unaffordable during the 1930s.

And as for this:

Also, a major result of the war was funding which went into research that lead to a lot of useful new technologies like radar.

The technologies that were developed for the war are indeed impressive, but what of the technologies that would have been developed had WWII not occurred? How would we know if the seen outweigh the unseen in this case? The previous question is not merely rhetorical.