The point is that TIE fighters were already designed almost 40 years ago, and now they are adding a function that it wasn’t designed to hold.
The Wright Brothers first Airplane were only designed for 2 people. What’s your point? Upgrading technology is a thing we do in real life so why is upgrading something not feasible in the Star Wars universe?
I wasn’t talking about the two seats, I was talking about the turret, the accommodation of which was not present in the original design, nor was in the new design. In the case you mentioned, those upgrades came along with accommodations to the original so that it can work. They didn’t just glue a chair to the back of the two seated plane, they changed the size and shape of the cabin along with the other dimensions to allow for more seats.