SS7 is not fixable in my opinion. It needs to undergo the metaphorical equivalent of being burnt to the ground and having its ashes stomped around on a bit.

The further you go into the architecture of the "trust based" PSTN, SS7, traditional Telco stuff... The more you will see the total lack of modern cryptography, PKIs, zero trust network modeling, etc.

I'll admit that my perspective is skewed by working in backbone IP network engineering for a mid sized ISP. We occasionally have reason to interact with some pstn related stuff. All of the real technical innovation, security advances and such have been taking place in the ISP world for the past 25 years, not the Telco world.