It really depends on the structure. Giant steel objects are much harder to keep intact, to the point where non iconic objects are easier to replace once than maintain for 100+ years.
https://www.australiangeographic.com.au/blogs/ag-blog/2013/1...
Rather than a large number of people painting it for a small period of time, which requires a startup and close down overhead; they have instead resourced it so that they can keep a small number of people employed permenantly who accumulate expertise and minimise the mobilisations costs.
Except you get exercise and fresh air.
Whoever downvoted me, thanks a bunch. Not sure why.
Running a team of painters still costs more than nothing. The cost of painting doesn’t cover the cost of ongoing inspections for metal fatigue etc etc. Engineering reviews, government oversight and so on.
Large bridges, especially with vehicle traffic, vibrate quite a lot and this is causes major ongoing concerns with fatigue, which means extensive monitoring along with the painting. More costs (not saying it’s unjustified).
Keeping a team going is efficient as you say, but more expensive than nothing.
As you say, it would be a lot of work to maintain if they wanted to keep Arecibo running.
For those that don't know, paint should be considered a non-optional component of most iron or steel objects, since paint serves the important role of preventing rust formation. It's less important in most uses of aluminum, since aluminum oxide tends to form a protective layer instead of flaking away.
With smaller boats labor is a higher fraction of construction costs vs raw materials. Going big results in massive increases in efficiency which almost paradoxically shift things to a shorter ship lifespan.
What about the Eiffel Tower? Or Blackpool Tower. That proves it can be done.
(Saying that, for example, 300-series stainless, around 30-40% chromium and nickel, "is iron", could be fairly criticized this way. Mild steel not so much, and that was what I really had in mind with the statement, although I concede I didn't say so.)
But if it is obsolete, then that is another matter.