I'm not too sure Rosberg's departure is best for the team. On their time together, Hamilton had weekends when he simply looked lost with setup (used to happen at mclaren a few time as well). On those occasions he benefited from Rosberg's data, tho the other way round happened more often i presume. So putting a young gun in the other seat can mean Hamilton could no longer look at a faster driver's data on said occasions.
I feel Rosberg's deperature is a loss to the sport in the immediate term. If Mercedes are dominant again with no one to challenge Hamilton, what then ? 2002 and 2004 weren't fun...