I was a member of the YUI team until a few months ago. I'm still at Yahoo now, just on a different team, but just wanted to give my own thoughts on this (I don't represent the company or the YUI team).
I'll be a bit melancholy today, but I'll raise a glass to YUI tonight. RIP.
Also, super excited about the work Yahoo has been doing with React. Keep it up!
+ New devs can soak up a ton of knowledge quickly.
+ New devs don't come with a lot of baggage and tend to enjoy trying lots of new things.
+ New devs brought new ideas to the team that often challenged traditional ways of thinking.
+ While veteran devs acted as mentors and shared best practices.
From a recruiting standpoint, it's also easier to find new developers than veteran ones. So many companies fight for experienced talent that they overlook inexperienced talent, which meant I could assemble a good team relatively quickly.
It changed the recruiting process quite a bit though. Interviews became more about assessing potential and ability to learn than existing skills (which I think is more important anyways). It's not easy to assess for these things though.
It also changed the training process and team dynamic. The environment was one of constant learning and collaboration. New ideas were welcomed, code reviews were frequent, everyone was encouraged to hold a quick informal brownbag session on something they learned (even if others were known to be the "experts"), formal mentoring programs were established, etc.
And it made the veteran hires even more important. These devs needed to not only be strong technically, but strong with interpersonal skills too. But someone who can do this and support an inexperienced team with lots of potential is worth their weight in gold.
A lot of good developers emerged from this process. And I should add that new != young developers. There were some devs who changed careers to become devs, and shared the same energy and ability to learn as recent college graduates.
Cheers to people like you!
You know... It's still open-source.
If not even Oracle could manage to kill MySQL, I seriously doubt there is a way to kill YUI.