I think that main problem of using old versions of php is the given ability to install old versions on your server (hosting). For example on my server I can choose between 5.2.17; 5.3.39 and 5.4.43. Of cause I have chosen the last version, the newest. And as there was written in the article about Wordpress, Drupal has the same problem. Drupal gives the ability for users to install CMS on php version 5.2 or higher. If you are not advanced user (programmer) you can easily make a mistake and configure your server with old php version. Then you can have problems with incapability with modern modules or scripts.

I'm somewhat surprised these days when I see a company using a shared hosting account. A t1.micro instance on AWS is trivial to set up and run Wordpress on, and cheap, and really fast. But I know why it happens: agencies. Most agencies will host your company site for you as part of the cost of getting the site made. That hosting is likely to be in the region of $300 - $500/year (compared to the $200/year for a big brochure site on AWS). The cost to the agency of a server with WHM and CPanel is no more than about $20/year per site - so shared hosting represents a vast amount of profit.