Silverstein created an adult version of the story in a cartoon entitled “I Accept the Challenge.” In the cartoon, a nude woman cuts off a nude man’s arms and legs with scissors, then sits on his torso in a pose similar to the final drawing in The Giving Tree in which the old man sits on the stump.
Jackson and Dell (1979) wrote an “alternative version” of the story for teaching purposes that was entitled “The Other Giving Tree.” It featured two trees next to each other and a boy growing up. One tree acted like the one in The Giving Tree, ending up as a stump, while the other tree stopped at giving the boy apples, and does not give the boy its branches or trunk. At the end of the story, the stump was sad that the old man chose to sit under the shade of the other tree.
From Wikipedia: https://en.wikipedia.org/wiki/The_Giving_Tree
It would seem to me Silverstein himself was clear in what he was trying to say, even if he was persistently misinterpreted.
Recall that he wrote the lyrics to “A Boy Named Sue” also.