I can't say for certain 9/10, but the general idea is correct. The mobile phase of the war in fall 1914 was the deadliest in terms of the number of casualties relative to the sizes of the forces involved. We think of the Somme and verdun as being meatgrinders, and they were, but trench warfare was low intensity conflict punctuated by intermittent high-intensity fighting. The idea of the western front being one continuous battle is misleading. In 1914, before the trenches, the intensity and pace of the losses was truly unsustainable, a large reason why mobility ground to a halt after both sides had expended themselves by November. The French and German armies were bloodied and exhausted, and the pre-war BEF was almost wholly annihilated.

Trenches were by their nature designed to protect troops. The wars of the later nineteenth and early twentieth centuries taught all the major belligerents the importance of field works, and they were well prepared to use them in 1914. I specialized in the British Army, in which a good amount of attention was paid to developing shelters. British commanders were quite concerned with the quality and efficacy of trenches in 1914-15, and units that half-assed trench construction were regularly criticized (a good example being Henry Rawlinson criticizing a brigade commander in October 1914).

So yes, while we think of trench war as being brutal, in terms of casualties, it was far more sustainable than the open combat of 1914. In other words, trench warfare, however awful it might have been for the men involved, was in many ways a response to the tactical realities of pitting flesh against machine guns and quick-firing artillery.

Sources: This is my area of research, so a ton. Good starting places are Strachan, The First World War and Murray, The Rocky Road to the Great War; for a more in-depth look at the British case, see Dykstra, "Entrenchment and field fortifications in the British Expeditionary Force, 1914-15" (MA thesis).