Briar is dead because it doesn't work on iPhones. It doesn't work on iPhones because iOS will only allow waking the app from background when there's a push notification. Push notifications have to go through Apple's servers, which defeats the purpose of a decentralized app where your messages (and metadata) can't be traced.

only insofar as it is 1. not illegal to do so or 2. the cost-benefit of violating such laws makes sense for the majority of users, who are not doing things that are actually illegal

because without such a critical mass of normal users you get something like tor or grapheneos that the state begins to associate with people engaging in unsavoury activity

No, because most people (not on HN) value convenience much higher than privacy.

Only, if they manage to improve it, so that regular photos and voice messages can be sent. I am mostly fine with texting only, but such things are an instant no-go for most people.