Indeed, it's incredibly selfish and naive to depend on security by obscurity here when you can significantly up the bar from basically one step above full-text all-access for every global spy agency... to a serious encryption system.
Regardless, the exposure ship sailed long ago anyway when WhatsApp with their billions of users, including plenty of terrorist groups in the middle east and africa, decided to adopt the platform.
Additionally, breaking crypto systems like this in practice, if possible, pretty much always means targeted attacks. Which is a significant difference from the type of mass surveillance which Skype et al currently allow.
Even Blackberry bragged about building a system for the NSA/CSEC to provide real-time data access to every BBM being sent in the Toronto area during G20. It's very likely that Skype has a similar system and this change (should) make that fundamentally infeasible.
I prefer for Signal to be attacked and to matter than to be unattacked and to be irrelevant.