WordPress will change its requirements to use a higher version of PHP when the following cases are met:
- A significant majority of users are running 5.3.0 or above
- A major release cycle release approaches
- The code/lead developers involved all agree the above two are fulfilled
Sadly, the first entry results in a vicious loop, and guarantees a significantly out of date minimum requirement that is only set to get larger now that PHP release cycles are speeding up. Hosts won’t upgrade PHP if what they already have works, despite the performance and security benefits.
For a shared host it pays to have slow PHP because it forces the end user to spend money on fancier hosting such as pay monthly VPS, faster machines, dedicated machines etc.
Some hosts can be good about this, but a significant number of people are neither aware or capable of complaining about it to their hosts ( some people think PHP is something you install in your browser like flash ).
It also doesn’t help that WP Core development is significantly focused on backwards compatibility, and are handicapped by their own success
So realistically, the minimum requirement will be bumped up when there’s little choice but to upgrade PHP at the host level, or some massive unavoidable bug is found that requires it.