I get that Signal is probably the best option for smartphones. And that maybe its vulnerabilities are only relevant for "TAO targets". But the problem is that "TAO targets" is in rapid flux, given developments in automation and AI. So arguably, more and more journalists and dissidents are becoming vulnerable.

And there's the fundamental insecurity of devices with cellular-radio connectivity, and operating systems that users can't control and lock down. Signal can do nothing about that. Even something as simple as reliably obscuring identity in connections to Signal servers is nontrivial.

You are implying that cost of TAO consists mostly of labor costs. Which is false. NSA and friends are not really limited by money. They are limited by amount of unpatched software vulnerabilities. Every use of vulnerability in the wild is a chance of revealing it to world and losing it. Snowden docs reveal the existence of automated software which evaluates chance of vulnerability being revealed by attack. XKeyScore or one of related pieces, AFAIR.

Does it know whom of your contacts use Signal?

Does Google know you've sent a message?

Does Google know that you are receiving a message?

Does Google knows who from your contact list send this message?

Can Google infer from pings who is communicating with whom?

Yes to all of those, because they have root on your phone.

Eh, that’s exactly what it is currently advertised as.

A tool, supported by Snowden, to be used by journalists who are at risk of being under active surveillance by state actors.

That is the very definition of a TAO target.

And for non-TAO targets, WhatsApp is just as good as Signal – the users won’t read the source code anyway, and in both cases President Trump gets your social graph.

Aka NSA's "we really need to get access to this device and will fund diverting backbone traffic / writing firmware malware / whatever else we need to in order to get it" team(s).