Nice write up. If the applications can work on 5.3, there will not be much problems to upgrade to the latest versions of PHP.
May be all library owners should consider upgrading the php versions of libraries and force people to update the apps to latest versions. Then there is a question of how to overcome the legacy applications. We should provide an alternative library which works with the latest version.
Eg : we have password_ functions for less than 5.5. Like that we should replace other functions which can make use of the 5.5 functionality .