I feel like the point of the Lord of the Rings wasn’t the destruction of the Ring and the defeat of Sauron, but the return of soldiers to their homes. I feel like Tolkien wrote everything before the destruction of the Ring so he could write about what happened afterwards.
Merry and Pippin are able to return to their normal lives. There were no shortage of men who literally grew while in service, put on a good diet and getting good exercise for the first time of their lives (note that Merry and Pippin literally come back taller). They saw battle, saw friends fall, and experienced the horrors of war, but they never saw the trenches. The war was on the whole a positive experience for them, the great adventure of their lives, and they came back to be the leaders of the next generation.
Sam and Frodo are the men who lived in the trenches for years. They walked through the craters of Verdun, slogged through the mud of the Somme, trudged up the ridges of Passchendaele. Their journey was through worst of the Great War. It wasn’t just the Ring that broke Frodo. And while Sam didn’t break, he certainly had deep cracks in him. Tolkien would have called it shell shock; today we’d call it PTSD. Frodo goes off into the west. His real world equivalents committed suicide.