I don't know if the idea that the powers were slow to react to the challenges of trench warfare. I think it was more they struggled to find a solution the problems trench warefare posed. The Germans tried the attrition of Verdun, the British tried mass bombardments and giving the men a football to kick at the Somme. It was not untill 1918 that either side evolved tactics that could get sustained success. I don't know if this ineptness or just the fact that it seems simple with hindsight.