If you want your software to never change, then sure, a proof of correctness is enough. But I've yet to see a piece of software that is both relevant and never has to adapt to new requirements.
The level of blasé disregard for basic secure coding makes me think it is on purpose. More attention to security has been put into Postfix and OpenSSH than any of these encrypted but in no way provably secure chat systems.
What language would you use?
The requirements are that it be low-level enough that you can zero keys from memory and needs to be able to be ported to every major OS, including iOS and Android.
The red herring that gets trotted out is timing and side channel attacks, crypto itself can still be done in nacl. But everything else, GUI interaction, network interaction and parsing of external data should be done in a safe language.
The same argument has been said for Java (but turned out to have worse problems in practice), Ruby (where if you every wrote something in Rails you've been vulnerable halv a dozen times by now) and PHP (and any commentary here should be unnecessary).
Sure, all the obscure languages you mention which never had even one project with a fraction of the usage base of Apache or Postfix may have no known security problems, but that's not a something to boast about. If you really want these languages to succeed, build a project with the potential user base of Nginx or Linux. Just don't complain about other's language choices, especially not when those are well known and well understood ones.
And for two large companies to be working on something as large as Servo, even before the language went 1.0, there must be something good about it.