I disagree most strongly about the need to dig up and destroy walkways. We have 3 dimensions to work with - to go up, or, down. In any case I would see this analogous to a public urban rail system such as a metro or a tram.

I don't see it as a big issue to dig a tunnel for this - lots of cities implement rails and such that go wherever public transportation is needed.

The only argument which I agree is the biggest hurdle - which is actually an unknown - is how to make the system sufficiently robust so you don't need an army of engineers to maintain it. The fact that there are no known robust systems is not a good enough argument. Lots of mechanical systems were in the development over a century after someone figured out how to make them as practical to make a major impact(steam engines, automatic weapons).

How robust is the most robust implementation possible and how expensive it is are the two major questions to my mind. If it's too expensive to maintain - then it's too expensive to maintain.