Quoting Dries Buytaert <dries.buytaert@gmail.com>:
Mark my words: a PHP5-only Drupal 7 will leave many, many Drupal users behind.
There will always be those that get left behind; that is even true if you stay with PHP4. They can always use an older version or the services of others to upgrade. Only weeks ago I would have joined you in your caution but I believe that the only way out of the whirlwind is to take a different path and the pavement is being laid. Now is the time to jump on the bandwagon driving down the new laid pavement. PHP 6 will be out soon enough and we're still toying with PHP 4. Go PHP 5, Go! Why are there always those that are left behind? One reason is fear. People are of afraid of change and what it might mean to them doing things differently or seeing things with a different view. Another reason is laziness. People setup the product and just don't want to be bothered with changing it. They like the way it is and just don't want to spend time upgrading. There are probably other reasons but as I see it, these are the top two. Earnie