a) only a certain elite group of people or companies will be able to use it (in this case, PSTN operators)

b) total trust between all parties using it, so there's no need for provably-hardened cryptography.

both of which are now laughable in a modern network security threat environment.

In this case SS7 was just never designed with the concept that malicious third parties might get access to it, or that it would not be operated by RBOCs (regional bell operating companies), or the international equivalent thereof (national run telcos such as British Telecom, Telecom Italia, etc).