Why did Grandpa Really die?

Yes you are right. One of the running themes in this story is the dignity of the farmer. When the Joads were kicked off their land, Grandpa's identity was destroyed. It isn't easy to re-invent yourself at any age. For Grandpa Joad a new start was impossible. The humiliation of losing his validation of his farm combined with fear of the unknown was too much for this old man to take.

yes he did...he died on the inside when he left the house but had a stroke after the fact