Err.. this isn't exactly news. From March this year:

Well, it’s true and it’s official. Today, I asked Twitter about Ev’s current role in light of Dorsey’s taking over the product lead. A company spokesperson sent me the following statement:

Ev decided a couple months ago to be less involved day to day at Twitter. He continues to have a close relationship with the company providing strategic advice and, of course, he remains an active board member.

http://techcrunch.com/2011/03/28/ev-less-involved-twitter/

Could it indicate that Twitter's hardest engineering problems have been solved and they plan on keeping the service lean and simple, so there's not as much need for a large team of people to build new features?

This would make a nice change from Facebook and Google, which seem to be straining under the weight of feature-bloat.

Or, is it symptomatic of Twitter hitting a dead end and senior staff deciding there's no money/business there?

This is all speculation, would appreciate any opinion or insight. Hypothetically, if you were offered a position at Twitter right now, would you take it?

Just because a few execs leave the company doesn't mean that would cause more fail whales.

The real value is the engineering and ops teams who keep the system going. I'd look to see who on those teams have left recently.

the board bought Jack back and he is trying to take over, it seems.

this happens to many companies when they get big enough and successful. look at msft post-gates (even during gates, but his authority would overrule deadlocks) and apple etc.

Twitter caught up with the curve for awhile, and maybe this is a short term issue, or maybe the curve got a bit more steep, I don't know.