Replacing the existing infrastructure which uses these signals will be much more expensive and time consuming than anybody realizes.

You can use the stars to set time too: https://www.esa.int/Our_Activities/Navigation/ESA_sets_clock...

To claim that the two radio stations in Colorado and Hawaii are our last grasp on time is a bit of a stretch.

There are legacy systems in existence that rely on those stations, but none really seem particularly critical or that can't be easily moved over to something more modern than church bells.

Not many stars in the middle of a building. Radio time isn't our last grasp on time but it is an efficient grasp that has been proven to work damn near everywhere for cheap.

Old != obsolescent and you don't automatically save money by turning something currently in use off you have to look at the cost to remove or replace it in all use cases.