Is there really a demand for bridges or tunnels at both ends of the Bosphorus or could they just build one?

The Bosphorus is unusual compared to any other strait I can think of in that it has a city that straddles the entire length of it, meaning that Istanbul has large population centers at both ends, and minor ones all along the length of it. The strait is still narrow enough for a bridge or tunnel at both ends, but does it really need new crossings at both ends just for the sake of the small communities within Istanbul that are far from the existing bridges and tunnels? Wouldn't it be cheaper and smarter to just build one and make everyone use it, even if it's a little more inconvenient for some people?