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.
If the choice was between (easy & insecure) and (hard & secure), you'd have a point, but there's plenty of easy ways to have secure communication: for example, OTR-over-(any IM protocol) is about as simple as it gets (it's literally a one-click thing, and can be set to automatically go secure with no user interaction), doesn't depend on a provider for keys, and can work with any IM network. If someone can install an executable file, they can install and use OTR.
Sure, it doesn't conceal metadata, but most (all?) IM networks leak metadata as well. XMPP-over-Tor-hidden-service can help mask that, and isn't really complicated for the users ("Open Tor, click 'Connect' and wait for the green light, then open your IM client.").
Tox is another option: anonymous, distributed, and with no single point of failure. It's as easy to use as any other IM client.
Even if secure communications weren't as easy as non-secure methods, there's plenty of easy-to-follow guides on how to setup and use secure methods. It's hardly rocket science, and those methods aren't going away, so there's no reason to expect that bad guys that are motivated to keep their communications private will avoid them simply because they may be slightly more difficult.
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.
Sure, a company could do that (and several do), but there's certainly a lot of interest from users to have secure systems (devices, accounts, etc.) that cannot be remotely unlocked or decrypted by the company or authorities (see Apple). Considering how massively the US Government abused its position of power and authority through massive, warrantless surveillance of people, hacking and snooping corporate networks, doing shady things like parallel construction, and generally violating everyone's trust, it should come as no surprise that there's some pushback from users and industry.
Statistically, the risk posed by terrorists is so low as to not be a concern in my day-to-day life. I'm in far graver danger from occasionally eating hamburgers or riding a bike than I am from terrorists. Considering that "free societies" are hardly permanent things, and that a major event or political upset can dramatically change the nature of government, I'm more worried about granting even the most trustworthy government (which the US Government is not) powers that groups like the Stasi or KGB could only dream of in exchange for the dubious assurance that (a) it's necessary for them to stop bad guys and (b) they won't abuse that power.
Your mileage may vary, of course.