"has electricity"

"what if they move, they can take it."

Do you not typcially have these in your area? No one takes them when they move; they sell with the property. I've only ever seen one that the owner sold and got moved to be used as a child's playhouse, and it essentially had to be rebuilt after relocation. That's why no one takes them when they sell.

"39 years seems right but crazy long given nature of the asset."

Unless it was cheaply built to start with, it will last at least that long.