You seem to understand "plot" on very superficial level. A book does not need a complex plot to be great. On the Beach by Nevil Shute is another one of my favorite novels, and on a surface level, it also doesn't have a complex plot; rather, the character interactions make it great.

The exploration of ideas is the actual substance, and doing that through characters that represent ideas, ideals and ideologies is exactly where the novel shows its strengths as an art form. Simply writing a sermon about your ideas is what would be lazy; there's nothing lazy about TMM unless your understanding of plot is simply that "things happen".