The ability for it to scale or not scale isn't very relevant if the product continues to have repeated security holes - especially those in default files and "wordpress owned" plugins and themes.

Wordpress also continues to face the problems of inefficient or poorly indexed queries, and a myriad of issues related to plugins, their coding, their efficiency, and of course the security that comes with it.

It's a pretty good product, but much like Google's Chrome, it is getting loaded down with more and more "neat features" and getting to be less and less what it is.