First, generally speaking, you learn by doing. Without the mistakes on the Somme, the British army will simply make them on there next big operations. At it was not just Haig, there were problems with many of his flag officers down through the ranks.
Here is an example from a documentary i watch. On the first day of the attack, a corp size element was attacking a division size German element. Two divisions forward, one division back. The plan called for the reserved division back to support the attack on the right. As the battle progressed, the first division on the right was slaughter, and driven back to the trench, but the division on the left took heavy losses, but occupied the German lines. The reserve division commander went to the corp commander and asked permission to support the division on the left, reinforce it, then attack original target through the flank. He was denied because it was "too early to deviate from plan". The reserve division was destroyed, and the corp commander was relieved after the battle of the Somme as were many officers who were not good at their jobs. Meanwhile, the Germans were organizing a reinforced regiment to try to retake the lines lost by the attack on the left. One of the battalion commanders realized the organization process was taking to long, so he simply attack on his on. By the time the German corp commander had received word that the battalion had attack without orders from above, the battalion had retaken the German lines. At the end of the day, the Germans had their original lines, and had trade moderate losses for one division for heavy losses for a full British corp. It takes combat for the cream to rise to the top.
BTW, while tanks did have success in 1917, they were also used badly. For example, one unit was taken into a masonry village, where an know German gun battery was at. The tank unit was cut to peaces, so not attack in 1916, means that in addition to lesson that need to be learned for tanks, they make many of the same mistakes they made in OTL in 1916 with infantry. It is not clear the tank attack would have gone much better.
Also, consider that without the attack in Somme, the Germans would have had additional reserves to use elsewhere. Someone more familiar with Verdun and the German GHQ thinking would need to provide some likely details, but my guess is that the French take a lot more casualties in 1916 and the British a lot less. So yes, British morale is a lot higher, but French is a lot lower. The mutiny/strikes by French soldiers in 1917 is likely a lot worse. There are also diplomatic issues with the British not helping the French in the hour of great need.
And if the Germans send enough troops to blunt Brusilov in 1916, and the A-H has a lot fewer troops capture, then A-H is a lot stronger, and this has a lot of butterflies for 1917 in the east.