While it may be true the shell of an older Airstream has a very long life, none of the rest of it does. Looking at restoration threads on the A/S forum, one will find that these old trailers often need extensive frame repairs, in some cases an entire new main frame needs to be fabricated.
As well as repair or replacement of functional parts like furnaces, air conditioners, wiring, addition of microwave ovens etc.
These repairs are very labor and time intensive to the point they are not really economically feasible. I applaud the efforts these folks put into the old trailers, but the reality is they are doing it for the love of the old trailer, not because it makes economic sense. They have more in just the restoration/refitting process than a new white box trailer costs.
Let's be honest here folks: people buy, use and restore these trailers because they like the unique, shiney, iconic shape. Not because they have any long term functional advantage.
Even if one were bought brand new and perfectly maintained, many owners would find the interior to be dated looking in 10 to 15 years, and would spend at least some money refitting the furnishings.
To each their own. You either want one or you don't, but potential owners should be honest with themselves if the dollar side of the equation is important.
EDIT: if a person wants to read an excellent thread on the resto of one of these trailers, go over to airforums, and search for millertimeUS thread on the resto of his '76 Tradewind. He started the thread in 2014, chronicles his restoration process. His project ended up very nice indeed. Lots of work, starting with a brand new custom fabbed main frame.