You need to define when and where
What if Operation Roundup Had gone down and failed?
You need to define when and where, and what the situation is in the Mediterranean Theater and in the Eastern Theater.
If ROUNDUP went into NW France as an OVERLORD or HUSKY analogue in July, 1943, against what the Germans historically had in NW France at the same time, then no, the odds are very much in favor of Allied success.
The thing is, an invasion in the West will only be launched at the time and place of the Allies' closing, which means there's really no way for it to fail. The Allies don't HAVE to invade, as long as the Soviets are still fighting...
The Germans, however, have to fight the Soviets, the Allies in the air, and prepare to meet an Allied invasion in NW Europe, the Med, the Balkans, or Norway - simultaneously.
Multi-front wars are like that...
Best,