Wow. I don't mind a bit of analysis, but you guys do tend towards overanalysing.
Once again I'm reminded of my favourite online alt.hist stories,
Wikihistory. If you haven't read it, here's a link:
http://www.abyssandapex.com/200710-wikihistory.html
It's short, it's funny, you'll like it.
Serious now. Let's take a hypothetical. You are given a time machine, a loaded pistol, and directions to Sarajevo, 28 June 1914. You can stop the assassins, including Gavrilo Princip, determined to kill Franz Ferdinand (not the band). They were fighting what they saw as a just cause, but you know it'll make a large portion of the 20th Century crispy around the edges.
But out of all the conflict, economic, military, political, all that friction has produced intense advances in technology, medicine and physics to name 3. I'd argue that events of that day are still echoing around the world, nearly a hundred years later, and the events that precipitated it continue to resonate too.
So. Yiou can stop it. There lies Princip in a pool of his own blood, as the Archduke is whisked away to safety. No World War 1, hence no Depression, WW2, atomic weapons, computing, triage, and so on. You can say those things would have happened anyway, which I can say is probaby true, but they would take much longer to happen in a peacetime climate of a much more fractured world - the Ottoman and Austro-Hungarian Empires would hang on. The Russian Revolution may or may not happen, and probably not. And that would be just the start of the massive changes between 1914 and 1920.
So it'd be safe to say that 28 June 1914 is a fixed point. To change that is to change
everything. Do we, you and I, have the right, or even the rudiments of the knowledge, to reshape the world like that? It could have been made worse, by delaying WW1, as military scientists build nastier machines* to make the Great War, when it comes, even more hellish.
While I'd like to stop millions and millions of deaths, do I have the guts to live up to the consequences.
You could also elaborate this into a 9/11 story. I'm pretty sure anyone here, given the same time machine, would go to NY, 7AM, 9/11/2001, and phone the WTC and say there's bombs in both towers. Or phone the relevant airports and say, "hey, these passengers are terrorists". But someone could come back from the future and say, "this event changed things, and it
has to stand". What then?
And of course another example is BTTF 2.
Messing with time to a big extent is incredibly tricky. Much as we might want to, we can't do it. And the Mars Mission of WoM is a similar kind of event, afaik.
And all that aside, it's teh drammah. Live with it.
*and there isn't a contradiction there - a 1914-18 war produced machines, explosives etc of a certain capacity, a (say) 1920-24 war would have produced nastier weapons,