What was the alternative? I can't see how a cross channel venture could have been launched in 1943, too few landing craft. Italy was at least doing SOMETHING, although all too little for Stalin's tastes. IMHO D Day was not possible in 1943 no matter what, and the Italian campaign had no effect on the resources available for D day in 1944.

Knocking Italy out of the war is a bit of a red herring though, once her fleet was taken out she really had no offensive capability (or manufacturing capacity) at all.

I reckon it was a basically irrelevant side show, handy for propaganda and learning from mistakes but strategically irrelevant.