Mobile homes generally have a useful life of 30 to 55 years before they get worn out and are no longer worth repairing.
Mobile homes can last a long time as long as they're maintained properly just like a regular home.
Mobile homes and especially the older ones have thin walls and cheap fixtures etc and are made with bad flooring materials that when they get wet they fall apart quickly.
If a mobile home is 40 years old and is worn out then I would say that it's not worth fixing since you'll never get your money back after spending the money to fix it up and trying to sell it.
Most times you can find another mobile home cheaply enough that is in better shape than your older mobile home that needs a lot of repairs.
Mobile homes are hard to work on too especially if you have any water line or other plumbing issues since the water lines and drain lines are all hidden under the insulation under the mobile home.
And to get to the plumbing you have to rip out insulation underneath the home.
I used to live in a mobile home and my neighbor who helped me fix the plumbing had to rip out insulation under the mobile home to get to it.