You know how, in Western story structure, there is often a refusal to answer the call of adventure/duty early in the story? The Big Lebowski is a story about what happens if that refusal lasts for the entire story, while also getting inadvertently shoved into an increasingly dangerous and strange world.
The Dude just wants peace and quiet. He already has his happy ending at the start of the story, and he spends the entire movie fighting to maintain it. More importantly, through the various characters he encounters, it’s pretty clear that there’s a certain wisdom being pursued here, not just a gimmick. The other Mr. Lebowski, who lauds his own wealth and achievement over the Dude’s constant nonchalance, is in fact a highly insecure and materialistic mess of a person. The nihilists are persistently static and unhappy, for the sake of proving a point on existence as a whole. Given that the Dude is also an aging ‘60s hippie living in the ‘90s, he’s essentially an answer to the question of what happened to the few hippies who didn’t just get a job, and “get with the program”, so to speak. Which, to be honest, is at least in spirit accurate — there are still a few hippie communes in a number of countries to this day, although their lifestyle requires quite a lot of daily work, but like the Dude, they don’t bother anyone and don’t do much. However, they focus a great deal on the small things in their lives, which maintain the peace and prosperity they have.