SSRS on 2008 R2 and its Database on 2016 Can we have SQL Server Reporting Service on SQL Server 2008 R2 and its Database(ReportServer and ReportServerTempDB) on SQL Server 2016? 

PS: Both are enterprise edition.
SSRS just needs a SQL Connection to a database that supports all of the features it requires. The connection type that it uses should be forward compatible to SQL 2016 (and even 2019).

I want to ask, why not upgrade SSRS as well though?