Wether it is called Istanbul or Constantinople depends on the scenario I think.
I mean, if the message is written by a turkish character, then maybe it should be Istanbul.
But if the message is written by a non-turkish character, then it would be Constantinople.
I find it illogical to call a City by its current name instead of its real name refering to a certain period.
It seems like, video game players are idiots and don't have any culture, and don''t care about it because "it's just a game". I found it rather depraciating for players and video games in general...
Besides, as it was said, the story would loss credit...
I wouldn't like a game taking place in 52 BC in Paris call it Paris instead of Lutetia just because some other game would do it... this is nonsense.
All this to say, Frictional Games rocks for doing their job very well