Hi,
MCSA on Server 2012 does not expire, MCSE certifications need to be re-certified every two years.
At some point in time the Server 2012 exams will expire but this will not be anytime soon, the Windows Server 2016 have not been even been announced yet so it safe to go for Server 2012. In general Microsoft also offer a upgrade exam to prevent you from
doing all the exams all over again and I expect that this will also be the case for the transition from Server 2012 to Server 2016
Danny