make a pretty strong case that World War 1 was the deepest, darkest pit in the history of warfare - a nightmare to trouble the dreams even of soldiers in World War 2 foxholes
A simple survey of the battles of the Second World War, El Alamein, Monte Cassino, Normandy, the Hurtgenwald, Stalingrad, Smolensk, Minsk-Bialystock, Kursk, Rzhev, The Dnepr, Operation Bagration, the Sieges of Leningrad and Budapest, the Battle for Berlin, Iwo Jima, Guadalcanal, Tarawa, Okinawa and New Guinea would suggest otherwise. In general, being a soldier 'at the sharp end' in Twentieth Century Warfare was hell on earth.
To rot for months in virtually the same location
Nope; British troops spent 40% of their time in the actual frontline trenches, with the rest spent in reserve, in billets in the rear, en route to a new sector, or even in GHQ reserve. more often than not, sectors outside major offensives were quiet. The French had a very efficient rotation system, the Noria or Water Wheel, set up to ensure constant flows of units in and out, and rest periods for fought-out divisions.
watching people die one after another with no change in circumstances, tactics, or strategy
Again, outside major offensives the western front was quiet; furthermore, especially in 1916, tactics did change and became far more advanced as time went on. Strategy was more difficult, but by mid-1917 materiel resources were sufficient for Franco-British forces to execute well-supported set-piece attacks.
And when it was finally over, the shelling stopped, and the soldiers went home, what rewards of history awaited them?
Home, there families, the feeling that they had accomplished something? In the United States, the Commonwealth and Britain, there was some prosperity in the early-mid 1920s.
The Spanish Influenza, the terror of Bolshevism and Nazism, the Great Depression
Bolshevism if you were in the Soviet Union; for soldiers in Poland, Finland and the Baltic States there was the knowledge that they had defeated that menace, and preserved their independence. Nazism only became dominant in Germany after the Great Depression, and were it not for this, the 1930s would likely have seen continued prosperity and postwar détente. Way to erase an entire decade!
No generation in human history was subjected to as many consecutive hells as that one, and it boggles the mind that it was pure bad luck: They were born when industrial weaponry collided with antiquated tactics and the political wreckage of the ancient world.
So we're going to ignore any positive developments whatsoever in human history that took place at this time? Okay, sounds fake, but okay.