Put simply, there exist plenty of systems and techniques that don't depend on a third-party who could possibly grant access to secure communications. These systems aren't going to disappear. Why would terrorists or other criminals use a system that could be monitored by authorities when secure alternatives exist? Why would ordinary people?
That's a really easy answer -- terrorists use these simple platforms for the same reason normal people do: because they're easy to use. Obviously a lot of our techniques and capabilities have been laid bare, but people use things like WhatsApp, iMessage, and Telegram because they're easy. It's the same reason that ordinary people -- and terrorists -- don't use Ello instead of Facebook, or ProtonMail instead of Gmail. And when people switch to more complicated, non-turnkey encryption solutions -- no matter how "simple" the more savvy may think them -- they make mistakes that can render their communications security measures vulnerable to defeat.
I'm not saying that the vendors and cloud providers ALWAYS can provide assistance; but sometimes they can, given a particular target (device, email address, etc.), and they can do so in a way that comports with the rule of law in free society, doesn't require creating backdoors in encryption, and doesn't require "weakening" their products. And of course, it would be good if we were able to leverage certain things against legitimate foreign intelligence targets without the entire world knowing exactly what we are doing, so our enemies know exactly how to avoid it. Secrecy is required for the successful conduct of intelligence operations, even in free societies.