I guess PHP 7 is big step forward which closes many doors.
Don't forget Debian has PHP 7 in sid/strech only. The unofficial way how to install PHP 7 on Jessie is trough dotdeb repository. Official release should be probably in next debian release in 2017 (no release date has been set).
In my experience there are also companies who runs GO on PHP 5.3 so with PHP 7 you have two options - maintenance two branches (5 and 7) or strictly say minimum requirements are PHP 7 ...
However I'm interested in to upgrade core to PHP 7 too.
IT Analyst, Independent seasoned Group-Office developer, Educator of web applications