PHP 7 is on the horizon for October of this year. It needs to be tested for compatibility, as there are some backwards compatibility breakers in there.
So by the time PHP 7 rolls around, we need to either
a) be confident that Moodle will run fine (and probably blazing fast!) on it, or
b) We need to update our required versions to put an upper cap on supported versions, as all our release notes and checks are 5.4.4 (or whatever) and higher.
If changes are needed, we may want to amend old release notes for no longer supported Moodle versions with upper limits.
I know it's still early on, but I want to make sure this gets on the radar, as PHP 7 is supposed to drop right around the time of Moodle 3.0.
You can see the accepted changes for PHP 7 here: https://wiki.php.net/rfc#php_70