Edgecrusher
King
Following this logic any ingame civilization would have to decommission every ship build prior to industrial age ships, since not a single sail/oar propelled vessel was rebuild into a destroyer/cruiser/whatever in real life.
Following that logic, yes that would be the case.
They can always be kept around. The U.S. Navy still has some sail powered frigates commissioned, although they obviously never see action. They stay in Ports and serve as museum ships to bring in some extra revenue.
Have them upgraded into "Museum ships", that, that when in a city, has an option to build a "building". much like Great Scientist/Great General. The building it produces could provide some culture/gold (tourism/history) Not much, obviously, but some.
The U.S.S. Constitution in Boston is an example of that, much like the U.S.S. Nautilus in Groton, CT or the U.S.S. Massachusetts in Fall River, MA
I mean obviously Civ is a game and historical accuracy will never be reached. I think though there is big difference between upgrading
1. An Ocean Going Warship in the Industrial age to a stronger Modern Age version with the same abilities.
and
2. An Ancient warship powered by oars, incapable of entering oceans to the first ship capable of circumnavigating the globe which is powered by sail.