I'd wager that most of the remaining PHP sites are wikis and legacy applications that are too painful to port.
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.
How are you getting that number? From the link you referenced, the percentages are Wordpress: 40%, Joomla: 2.6%, Drupal: 1.7%
It's getting ever harder to find good PHP developers.
PHP will be around forever, but it's getting less popular.