The problem is like what happened in the novel Red Mars. If it breaks then the cable wraps itself around the Earth going over other countries. No one is going to allow something that risky to be built. For example falling CNT might short out power lines, crush things it landed on, as well as the cleaning up job of getting rid of tens of thousands of miles of cable.

One issue is whether the cable would fall straight down or wrap itself around the Earth. The consensus I recall from a forum on this subject is it would wrap around.

There is no reason to believe it would break up into pieces or air friction would slow its descent much more than a deorbiting satellite. Look at the controversy and panic they cause every time one of them deorbits.

Breakage is a major problem because meteorites would be expected to hit it regularly. It is anticipated the ribbon could be moved to avoid satellites though apparently collisions are not a likely problem.

The best shape of the ribbon from the forum seems to be a kind of net with large holes in it (making it much wider), so damage from a meteorite is less likely to cut much of the ribbon.

The best cross section seems to be an arc shape, maybe something like a quarter of a circle. A full circle would tend to get an entry and and exit hole so twice the damage. A ribbon would probably tend to curl into an arc cross section anyway like any flat ribbon does under stress.

One idea I suggested in the forum was to use the ribbons as power lines from an oribiting solar power station. You would use multiple ribbons for this far enough apart so they don't bump together. Apparently from memory over half the power would survive the journey.

Beaming energy down also has power losses. Using the ribbons as power lines also helps the vehicles travelling up it to siphon off power, otherwise they usually talk in terms of beaming power to them.