Nobody would lift up A Day in the Life as an influential masterpiece without Paul's verse.
The lovely, downtrodden verses from John are, of course, the backbone of the song. But the switch to Paul illustrates the existentialist perspective of the song, the idea that our daily concerns are fairly insignificant in the grand scheme of things, that no matter how deep into our own sorrow we may be, there's someone else just going on about their business like nothing is wrong. Connecting the two narratives, the tempo change then cleverly colors John's verses, leading us into the song's iconic crescendo.
A Day in the Life has groundbreaking songwriting because it considers tone and mood as an aspect of storytelling and utilizes the studio as an element of the arrangement. With a little creativity, you didn't have to stick to one through line anymore! If all we were given was John's story, it would certainly communicate the daily life of one man, but the way it juxtaposes two of them says so much more about the human experience.