> That means replacing those cables again in a few decades will be basically free, too.
I think you and me have a different definition of free. There are still substantial costs in both labor and materials to replace the outside plant, even if there is existing ducting in place with free space.
> Almost all of the costs of replacing the cabling in the US today is digging a trench, or getting access to privately owned telephone poles.
Firstly, if you have existing ducts there is no need for trenching. Secondly, a lot of trencing is replaced by directional boring and, where possible, by microtrenching or plowing. Thirdly, gaining access to poles isn't usually the problem. What takes time and money is to get them into such a state that new cable can be installed. Fourthly, your largest cost is always going to be labor, regardless of what you do. Lastly, even ignoring all the other costs there is still equipment and materials, which will account for at least a fifth of the project costs.
So in summary, no almost all the cost is not in the line items you specify. A lot of money is going to get spent no matter what when you replace your outside plant.
> As evidenced by Google Fiber’s struggles.
Google is also being actively hampered by competitors that own the poles they need access to.
> If that cost is eliminated, replacing fiber outside is as cheap as replacing it inside a datacenter.
That's a bit optimistic, as the outside plant is far larger than any datacenter, not administered by a signle party nor a controlled environment.
I think you and me have a different definition of free. There are still substantial costs in both labor and materials to replace the outside plant, even if there is existing ducting in place with free space.