The germans were close to achieving the bomb:
http://www.pbs.org/wgbh/nova/military/nazis-and-the-bomb.html With a little more time, and without the distraction of war and our related efforts to sabatoge their program, they would have succeeded. They were vastly outnumbered because Hitler was a blundering strategist. Had he died early of Parkinson's he would have been entombed and glorified, and Göring would have picked up the banner and carried on his ideology. Unlike Hitler, Göring had real military education and had a better grasp of military matters. If he had taken over, the world may be much worse off today. But yes, we could go round and round with what-ifs.
You don't think tragedy is necessary, but what about the US Civil War? Was that tragedy necessary? There are cases where tragedy is unavoidable, where these tragedies must play out before progress can be made. In a hypothetical utopian world where people are not selfish, bigoted, and small minded then yes, tragedy would never be necessary because groups of people behaving badly would voluntarily change and we could all move forward together in peace and progress. But that world doesn't exist. History is rife with examples of regimes and ideologies that could only be corrected through tragic means.
A few things that necessarily came out of WWII:
- As already mentioned, Europe finally started to unify and stop warring with one another. I would go as far as arguing that the European Union would not exist w/o WWII.
- The United Nations and resulting improvement in international relations. Who knows how many tragedies this alone has prevented.
- War Crimes Tribunals and the novel (at that time) idea of holding leaders accountable for crimes against humanity.
- An end to isolationism in the US and a determination around the globe that we can never let this happen again
- The Cold War (itself a tragedy) which was instrumental in bringing about: modern computers, space exploration, satellites, GPS, the internet.
EDIT: What's striking about this [partial] list is that these are things that create the conditions we enjoy today, where in most cases we can avoid tragedy because we have the necessary tools such as the UN and war crimes tribunals and world governments that are willing to step up and take responsibility in the world to prevent such horrors. The world pre WWII was a very different place, I think we easily lose sight of that.