Denial of service is one threat. All the others can be addressed with end-to-end encryption, which is becoming the norm on the Internet and should already be for sensitive government operations. Telephony for the general public is practically unencrypted, but that's because our spooks like it this way and have made sure that encryption remains inconvenient if not illegal, and thus they are responsible for this vulnerability.
As for standards, they are built on a baroque foundation of legacy telco crap designed by C-team standards committees, leading to grossly vulnerable protocols like SS7 (in addition to the laughable lack of security, the network also crashes if it is pushed above a certain traffic threshold). In practice, because they are so sloppily specified, interoperability requires access to the other vendor's equipment, which they make sure is not available to potentially disruptive new entrants. In the case of 5G, the 4G already deployed is predominantly Huawei, and since modern networks are essentially software-defined, they can mostly be upgraded but Huawei will do so only if you stay with them.
It would be best if 6G were totally software-defined to work on white boxes and got rid of the legacy ITU cruft, but chances are low.
As for telcos monitoring their network traffic, the author's naive faith in their technical competence would be charming if it weren't misplaced. Read Bert Hubert's excellent paper on how they have been so hollowed out technically through outsourcing: