Before we discuss the ending, how about we read the ending? Real quick, won't take but a second:
The time had come. / SO… / Marvin WENT. (87-89)
Now let's consider the beginning of the story. Again, just another second:
The / time / has come. (1-3)
Notice something change? How about the switch from has at the beginning to had at the end? It's a small, one-letter alteration, but as is often the case with language, one letter can make all the difference.
Why did Seuss bother with the change in verb tense? To signal a change in time? Exactly.
And with that change in time comes a change in Marvin. At the beginning, in the present tense, Marvin refuses to leave. Just look at the stubborn frown on his face. But once he realizes the time to leave has truly come, then we move into the past tense. It's in the past now because Marvin's already on his way, going wherever he needs to be.
Just look at how much happier both the Narrator's hand (no tense, pointy-finger) and Marvin are in that final illustration. If we consider Marvin as a certain historical president—let's call him R. Nixon—then maybe Seuss is slyly suggesting why the president should step down. Maybe both the president and all in involved will just be happier to see him leave.