You increase staff in the lead up to a new edition, you don't reduce it.
Unless I'm mistaken, we're only talking about one person - Bill Slaviscek - and it may be that if they want to take a different direction and are unhappy with how 4E has done then it makes sense for him to be gone.
Based on WOTC's edition churn in the past, I'll be surprised if 5.0 is released later than 2013.
Given that the gap between 1E and 2E was 10-12 years, 2E to 3E 11 years, and 3E to 4E 8 years, it would make sense that the gap between 4E and 5E would be 7 years or less. Furthermore, the gaps between edition and "sub-edition" has decreased - from six years between 1E and Unearthed Arcana and 2E and Skills and Powers, to three years between 3E and 3.5, and just two years between 4E and Essentials (well, two and a half really). The gap between sub-editions and new editions has been a bit more stable: 4, 5, and 5. But if we take Essentials and add 4-5 years we come again to 2014-2015.
If I had to bet money I'd say 2014 but I wouldn't be surprised to see it come out in 2013. I also wouldn't be surprised to see them take a similar approach to Pathfinder's alpha and beta testing, with an Alpha edition coming out in some form or another starting in late 2012, Beta in 2013, and 5E in 2014.