From the article:

Existing Themes

The biggest asset of WordPress is the large number of themes and plugins available for it. Of course, by switching to a templating language such as Mustache, the existing repository of PHP based themes are no longer valid. To take care of this, Automattic (the company behind WordPress) could bundle a plugin with WordPress 4.x that allows loading of old templates, which would be disabled by default. Keeping that code out of core would help keep the overall project more manageable and secure.