How long does a yacht last?
Usually most conversations on the topic center around the choice of hull material and how each material deteriorates over time.
Instead I suggest that a yacht will last as long as someone loves it.
It has little to do with the impact of the environment and more with the impact (or lack of) of the owner.
Hulls on the whole last very well, what does not last so well is all the machinery and electrical stuff in the yacht. What lasts even less well is the interior as it gets used by people or gets to be old fashioned, dirty and tired looking.
Interestingly the interiors of airplanes and buses wear out and are replaced several times before the structure or chassis wears out.
The answer to the question how long does a yacht last is; your yacht will last as long as you are prepared to love and care for it.
The yacht you bought is as good as the love and care of the builders, engineers and owners before you as they went about building and maintaining your yacht.