They know that the smart, major terrorist groups will …
They know that the smart, major terrorist groups will never use systems with government-mandated backdoors for their important communications, they’ll continue to use strong systems developed in and/or distributed by countries without such government mandates, or their own strong self-designed apps.
So it seems clear that the real reason for the government push for encryption backdoors is an attempt not to catch the most dangerous terrorists that they’re constantly talking about, but rather a selection of “low-hanging fruit” of various sorts.
Inept would-be low-level terrorists. Drug dealers. Prostitution rings. Free speech advocates and other political dissidents. You know the types.
That is, just about everybody EXCEPT the most dangerous terrorist groups that wouldn’t go near backdoored encryption systems with a ten foot pole, yet are the very groups governments are loudly claiming backdoor systems are required to fight.