If anybody develops a truly secure messaging system, free from NSA's snooping, it won't be Yahoo. Simply because Yahoo is US based and can be secretly forced to change the code or push a hidden update that will decrypt everything.
And by the way, such systems already exist
1) https://crypto.cat (after a few bad releases it looks like they got it right... probably)
That's why it would need to be open source and available for any security researcher to go looking around.
The reason I chose Yahoo, was that they have the marketing clout and the brand to pull this off. This needs to be something that really replaces email, and kicks in into obscurity. Otherwise it is just another hacker tool used by a minority of privacy conscious geeks like us.
They can be forced to change their code, affecting their endpoints, and acting as a wiretap yes. But in the OP's view he said it was an Open Source protocol, which wouldn't get a lot of "forced/secret patches" because it's too easy to see.
They could also spend billions buying every block of lego to create a Yahoo logo visible from space. Which makes about as much sense as introducing significant legal risks to their business all for a few thousand new email accounts.
And by the way, such systems already exist
1) https://crypto.cat (after a few bad releases it looks like they got it right... probably)
2) https://github.com/ChatSecure for iOS
There are a few more:
https://en.wikipedia.org/wiki/Off-the-Record_Messaging#Nativ...