Cuba has famously kept a bunch of very old cars running for a very long time. Does the environment there help with the durability of the bodies? There’s no road salt (no winter), but perhaps there’s a little salt spray in the air, since Cuba has a high proportion of coastal area. So I’m wondering how it all plays out.
If no data is available on Cuba, then how do cars fare in Miami? Do they rot faster than they do in, say, Minnesota?
As to the latter, no, because the roads don’t need salting/gritting down here so there is little or no underbody rust.
Most are kept running but up close are pretty rough. Here’s an Autoweek article from a few years ago.
“Bondo is everywhere; roadside repairs are the norm. Radios and instruments are long removed, and many cars sit on higher and harder suspensions than new. You’ll see old Caddy or Austin jacked up on any given side street, surrounded by an array of tools, the owner’s feet sticking from underneath.”
Probably not much different than cars in other nearby carribean islands. Puerto Rico would be a good comparison point.
One thing to consider is that in the US old cars are often junked once they become more expensive to fix than they are worth. For example, completely good working cars are often titled as salvage vehicles (making them practically worth little in the used market) when airbags deploy, as airbag replacements often cost more than the value of the car. Not an answer to the OP’s interesting question, but something to be considered in discussing the issue of old cars in Cuba.
Uruguay had only old cars until the 1980s. There were stilll cars from the 1920s used as daily drivers as recently as the 1990s. Due to the restriction on new cars, Uruguayan mechanics were very skilled at making replacement parts for cars, and very few vintage cars still had very many original parts. Most parts that wore out were replaced with makeshift workaround. I would expect that a lot of Cuban cars that are still running would have received similar maintenance.
As I recall, all the taxis in Monterrey, Mexico, were pre-1930 (or so) cars when I was there in 1967, I think as a municipal taxi license restriction. They were fun to ride in, and still very serviceable as town cars, but never driven at highway speeds.
I was told in Uruguay that the Model Ts were of no collector value because first, they had very few original parts, and second, the cost of shipping one to America would be as high as buying a restored one stateside. They were very cheap in Uruguay, simply priced as used cars. They were nearly all gone in Montevideo, but still very common in the countryside.
Old cars stand up very well unless there is a specific condition that militates against their longevity, and salt would be a particular issue. Cars rust out very noticeably in Texas gulf coast cities like Port Lavaca and Port Isabel, for some reason, but I’ve never heard of the same thing happening in places like Tampa/St. Petersburg.
So are the Thesean cars still the same old cars they used to be? 
I don’t know what Thesean means in this context. Is my wife’s '95 Subaru the same old car it used to be? A car is what it is to a person who has an interest in it.
It’s a reference to the Ship of Theseus. You might know it as the question of the broom.
There was a thread (recently revived) from 2007 - an AMA from a Cuban defector.
Of course, the money-grubbing Gringos say the hell with starving people, how much for the '55 Olds?
He replied that the were like the Frankenstein Monster - Chevy body. Lada engine, Fiat transmission.
I suspect the were all “30 footers” (look good at 30’, get closer and they don’t look so good).