At the beginning of 1813 I would agree with you. Berthier and others advised Napoleon in January 1813 to pull half of the troops out of Spain and send them east against the Russians and Prussians. That very well could have changed the war altogether. In the end, that's what Napoleon had to do anyways.

However, the revisionism taking place here trying to turn Spain into the main or decisive theater of war is an incorrect assessment of the strategic situation in Europe. Undoubtedly the British presence in Spain helped, but it was not the main or decisive theater.

And what also has to be remembered, without the Spanish guerillas, Wellington could not have maintained his army in Spain and Portugal. It would have been defeated and driven out of Spain. Likewise, the Spanish guerilla movement would have been defeated without the presence of Wellington's army in the Peninsula.

Sincerely,
Kevin