For a real-world example of the problem you're describing, China's Salt Typhoon attacks compromised lawful intercept infrastructure in the USA to engage in espi

For a real-world example of the problem you're describing, China's Salt Typhoon attacks compromised lawful intercept infrastructure in the USA to engage in espionage. A mandatory backdoor in Signal would be at risk from similar attacks. https://en.wikipedia.org/wiki/Salt_Typhoon https://en.wikipedia.org/wiki/Salt_Typhoon