"Popular," and even "ease of use," are not relevant for the label of Gold Standard when we're talking about security. Functionality for purpose is relevant, but if we're allowing for weaker security in trade for ease of use then I'd say just use SMS; sure, it's not as secure as Signal, but it's a lot easier.

Reductio ad absurdum aside, there are by my count about a half-dozen systems which are more secure than Signal. Systems which don't require you to give up your phone number, or publish it, or leak other personal metadata. You mentioned one, Briar, and there's SimpleX Chat, Tox, and Jami (the latter two have been around for a few years, and IIRC Jami's been audited). There are any number of apps (web and mobile) that claim encryption and anonymity such as Confide, Onion Chat, ChatS, Speek!, Peekno, and Threema. Ocelot and retroshare.io are peer-to-peer with no central servers, and are probably (metadata) secure.

I wouldn't call any of these individually the gold standard, but several are obviously more secure than Signal.

I can't get over how any system that required such a tracable and abusable piece of PII as a cell phone number could be considered the gold standard for privacy.