"If Edward Snowden and Bruce Schneier are going to spout the virtues of the app, I expect it to actually be secure when it matters - when vulnerable people using it to encrypt sensitive communications are targeted by smart and powerful adversaries." Because if the adversary is, say, an abusive ex that happens to work for the telco, for example, then it doesn't matter. Unless you're actively hunted by a G7 country you…
How do you defend abused spouses in discourse by comparing their needs to people hunted by the most powerful political forces? Surely these two cases ought not be on the same table for comparison.
Signal comes from recognition that very few people can practically operate high-effort tools, and if the effort to operate a fancy tool distracts them too much from the things they actually aim to do (it's rare that someone's goal in life is "using Matrix"), they'll fall back on something that doesn't distract them.
For example, even when facing a state actor — if training your cadres to securely operate an encryption tool takes away too much from the core activism of your organization, the tool is no longer helpful. Same applies if the actual overhead of using it ("both have to be online" requirements, for example, or for somewhat lower threat profiles, "no emoji") makes it impractical.
For most people the threat is going to be someone they know, or high-volume-low-effort attempts. Whether it's an ex or a boss or even most states, it's unlikely they're facing NSA. They often have a lot of other things to do, and neither their ability nor want to enforce technical solutions on others are high. That's the range Signal-backed apps generally target.