Italy was in a civil war from 43 on so there was already a foothold for Allies into mainland Europe.
But D-Day was a part of the Tehran agreements that also had USSR joining the war versus Japan after a German defeat. So the failures and victories of the Soviets and Allies cannot really be severed or separated… (credibly).
While you’re correct the advance would have been slower, bloodier, and with a more dubious outcome, we CAN surmise that the Allies failing to secure more of mainland Europe would have easily meant more area for the Soviets to have occupied in the vacuum inevitably left by the collapse of Nazi Germany.
Might’ve had a North and South France instead of an East and West Germany.
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