Alas, the simple fact is that the July 20 conspirators were poorly organized, indecisive, and completely misunderstood how the Allies would deal with them if they did suceed in killing Hitler and taking power. By mid 1944 Germany's (not nazi Germany's) fate was sealed. The Allies would not negotiate with them other than to affect a complete unconditional surrender - including to the USSR. Almost all of the plotters believed (if they thought about it at all) that they would be able to negotiate an "honorable" peace with the Wallies once Hitler was removed and save themselves from inevitable and justifiable Russian retribution. That ignores that fact that US and British policy makers and propagandists in 1944 saw Germany itself as the enemy, not just the Nazi party. This of course changed in the post-war period, but was not the wartime view.
Well, yes, the sad irony is that if they had assassinated Hitler, they would have been left with no other choices than either surrendering unconditionally (which I do not see happening, at least not outright) or being force to continue fighting, which is something that could go either ways:
The great advantage that they would not be bound to Hitler's insanity and impracticality (and, especially in the last months of the war, total delusion). This means they will probably not fight on beyond anything reasonable like under Hitler. However, OTOH for the same reasons they might perform better than OTL. For example by pouring soldiers into halting the Soviet advance in the East instead of pouring them into the Ardennes Offensive: after all, the Americans did slow down even without the Ardennes Offensive, perhaps forcing the Americans to take Berlin. The effect is that this might prolong the war by weeks, if not by months. The latter is extremely problematic because this could mean that the atomic bomb is dropped on Germany after all. There is also the chance that Germany grows extremely desperate and unleashes it's chemical weapons on the battlefield, which could make WWII take a far more gruesome turn, as the Allies would have returned that in favour, which would dramatically increase civilian casualties from the war.
Plus, post-war there is the (although slim) possibility of a new Dolchstoßlegende arising.
I should add, while none of the above scenarios is inevitable, the key is that Germany could have fared worse if Hitler had been successfully assassinated.
The only context, realistically, in which Germany could still yield a better outcome (ie, a peace on conditions) is that Stalin dies before the
Tehran Conference. However, what I do not know is what butterflies that would have on the outcome of the war itself.