What disenchants Gulliver with the notion of being immortal?

Gullivers vision assumes eternal youth. However the Struldburgs age and at the age of eighty, they are declared illegally dead. Their possessions are passed onto their descendents, they live on a small government pension, and their two focuses seem to be “envy and impotent desires” unless they are fortunate enough to become senile

Explanation:

from the excerpt from the "the dark game" the reader can infer that folke cronholm was spying for the allied nations.