Common-ish arguments for monopolies, as applied to this situation:
* potentially the interoperability you indicate
* infrastructure investment leading to stability
* similarly: best in class client side software
* security / privacy guarantees (yes. I know, ironic, etc. but the fully distributed multi-company alternative is likely worse on these dimensions)
* single point of accountability for the state and law enforcement. (yes. not likely a HN concern. Still valuable to the state and potentially regular consumers.)
* general pro monopoly argument: fewer resources are wasted in competition, and so can be applied to product development and research. i.e. bell labs
IDK how I feel the scales tip in this case, but treating it as cut and dry feels a bit naive.