Can't happen. Mission creep. The British had invested too much into preparing for the offensive and couldn't rapidly move to another sector, while the French were screaming for help because the pressure at Verdun was overwhelming them. They needed a major offensive to take pressure off the French and to work with the Brussilov offensive, which altogether was thought to be able to break the Central Powers in 1916. They knew German reserves were limited, so by pressing on all fronts at once it was thought they could provoke a breakdown in the German front somewhere, as the strategic reserve would all be committed. So even with the heavy losses on the Somme, strategically that was insignificant to the overall war plan, as it was thought that even with the Germans inflicting heavier losses their more limited manpower would give out before the Entente in 1916.
Though this particular book has a lot of problems framing the discussion and talking about casualty ratios, in the end it does make a good case that the Somme was as necessary evil that helped break the Germans down in the long run and as part of the 1916 situation is was critical to the Entente war strategy:
https://www.amazon.com/Bloody-Victory-Sacrifice-Twentieth-Century/dp/0349120048
Point being: the campaign could not be called off until it ran it's course.