Why we should be grateful that Hitler's assassins failed
For the greater political good, it is paramount that the July Plot of 1944 failed. Stauffenberg was a brave man worthy of our respect, and Nazism was an abhorrent dystopia guilty of the most atrocious crimes against humanity that the world had ever seen. However, Nazism was also a quasi-religious philosophy that had seduced a generation of Germans. A key component of its appeal was the 'Dolchstoss' Myth - The Kaiser's armies had never been defeated in battle, they had been stabbed in the back by Socialists and Jews at home. It was the Dolchstoss that embedded a sense of victimhood in the psyches of the German people.
The attack on Hitler would therefore only interpreted as as yet another betrayal by nefarious forces threatening to destroy the Reich. Hitler would be enshrined as a martyr, and the Nazi will to continue to resist the Allied Armies would only be renewed with even greater fanaticism. Nazism's spell over the German people could only be broken by the fall of Berlin to Stalin's forces.
In addition to a resurgence of Nazism, based on the founding principles of the DAP, the war would not have neatly come to an end in July 1944. There were too many obstacles to a peace. Stauffenberg and conspirators were not on 'our side', they were fervent German nationalists who foresaw the destruction of the regime, and wanted to salvage something from the ruins. For example, Goerdeler envisioned a post war Germany that retained Austria and the Sudetenland, such demands would have significantly complicated an armistice.
The surrender of Germany would have also accelerated the Sovietisation of Europe. With Germany demilitarised, Stalin's hordes would have swept into Western Europe and brought the war-torn continent under the oppressive and undemocratic rule of Moscow.