As the largest CMS on the planet, WordPress should lead the charge in forcing the use of supported PHP versions.

Use of new PHP versions benefit the users first, and whoever said that PHP upgrade benefits developers only, has no idea what he is talking about. Newer PHP versions are faster (faster PHP, more pages can be served with the same hardware, and users don't need to upgrade to faster hosting), and PHP 7.x is a lot faster than PHP 5.x. New versions of PHP are more secure (in this day and age, that should be a high priority for everyone), and they will stay secure as long as they are in development. When PHP reaches EOL, no more patches for security issue are coming, and no one really knows what kinds of security-related issues PHP 5.2, 5.3 and 5.4 have right now, they are unsupported for years.

On Dev4Press, I have set PHP 5.5 as the lowest PHP versions I will use to test my plugins with, and I will fix any bugs related to PHP 5.5 or newer only. Most of my plugins work with PHP 5.3 and PHP 5.4, and I know that some users still use these PHP versions, and my plugin might continue to work for years for them, but I am not going to test or fix issues with PHP older than 5.5.

A similar thing can be done with WordPress at first. Stop testing with PHP 5.2 and 5.3. Considering how slow WordPress development is, I don't expect to see a lot of code changes in the next major versions that will immediately break on PHP 5.2 or PHP 5.3. But, spread the word, increase minimum PHP required version, and I am sure it will have effect: hosting companies that have stuck to old PHP until know will be forced to deal with it, or lose users that use WordPress.