You are not seriously comparing Kirk and co. to Abraham Lincoln?
Bringing life again to a real historical figure through the medium film is something else entirely from telling stories about fictional characters.
Yes, I'm seriously comparing a character that you "can't" write about because his established future path is already known with a historical character whom, I guess, you similarly can't write about because his future path is already known. Many of our viewers don't differentiate the kind of immutability of the fictional Trek universe with the kind of immutability of the actual universe.
But you'd need to take it up with them regarding why they think that way.
Who said you "can't" write about anyone?