Plus, said catwalks need to be accessible, so how many elevators are you planning on putting in and maintaining now?

Pedestrian bridges flat-out suck. Better to get rid of as many vehicles as possible and slow the remaining ones waaaay the hell down so that people can walk along streets and across intersections safely.

If this idea were viable, someone somewhere in the world would have already done it. But the closest I can think of is Las Vegas, and that's only along one road, and it's not even a full pedestrian pathway but instead just some bridges over intersections, and those are primarily there to prevent drunks from getting run over by cars.

Hong Kong does this in the downtown area. It’s viable because of the very high population density and prevalence of foot traffic.

https://duckduckgo.com/?q=hong+kong+pedestrian+bridge&iax=im...

It works out great. Though admittedly it’s more of a workout than walking in flat Manhattan.

That's a several orders of magnitude difference between the capacity of the two systems.

Consider further coverage and commentary such as

https://www.academyofurbanism.org.uk/streets-in-the-sky-the-...

https://failedarchitecture.com/the-downfall-of-londons-stree...

https://www.architectural-review.com/essays/campaigns/notopi...

The closest thing I can think of is a series of connected pedestrian tunnels that you see in some Canadian cities like Montreal and Toronto. Those primarily exist for a different reason; winter is months-long and so cold that you don't want to step foot outside at all. Pedestrian bridges don't accomplish this goal.

For Minneapolis, the first segment was built in 1962. Segments are often retrofits, as many of the buildings predate there system.

Quite a few cities do do this (Florida, I'm looking at you!). It's a terrible design, might as well have a machine that punches all pedestrians in the face as they go by.

I assume it would cost more in building and maintenance costs than sidewalks.