The only reason that things like canals get built is because people can charge money to recoup their costs. Many infrastructure projects get built and don't earn their capital investment back. (One might argue most) The reason people still build them is a few are enormously profitable.
Once "going the long way" gets significantly cheaper, the folks in charge of the canal will lower their price so that equilibrium still means going through them.
The only reason that things like canals get built is because people can charge money to recoup their costs. Many infrastructure projects get built and don't earn their capital investment back. (One might argue most) The reason people still build them is a few are enormously profitable.
Once "going the long way" gets significantly cheaper, the folks in charge of the canal will lower their price so that equilibrium still means going through them.