[vayup]

Counter argument: When the sole reason for existence for Signal is private/secure messaging, it makes sense to resist opening up to third party development.

That's a big can of worms that invariably will impact their ability to deliver on their main mission - private IM. Eg of problems: Who gets dev access, how do you vet plugins/aps from deceiving users, would users understand the risks, when an app gets compromised how to fight malicious campaign to discourage using Signal etc. etc.