I'd wager that most of the remaining PHP sites are wikis and legacy applications that are too painful to port.
How are you getting that number? From the link you referenced, the percentages are Wordpress: 40%, Joomla: 2.6%, Drupal: 1.7%
The article even concludes with: "PHP is far from dead, no matter what any disgruntled developer may tell you. After all, 79.2% of all websites in the world can’t all be wrong, and most importantly, PHP’s market share has remained relatively steady throughout the last five years (oscillating between 78–80%). Few programming languages command that type of staying power."
PHP powers a lot of legacy code bases, yes. But there are also a lot of job opportunities in Laravel and Symfony which are both very good frameworks.
It's getting ever harder to find good PHP developers.
PHP will be around forever, but it's getting less popular.