The backend version of Flash
Am I the only one who feels Wordpress is the backend version of Flash, regarding reliability and security? Very very popular, seemingly irreplacable, fit for many purposes (and used for still more outside that category), brought to life mainly by absence of any better alternative (PHP being virtually the only language availabe everywhere on the cheap or for free) and so on.
Let's hope that the recent huge increase in availability of alternative runtimes - today, one gets a VPS or a cloud node for less than a PHP hosting instance ten years ago - will help us repeat the glorious fate of Flash on the backend.
We'll know we are there when the number of vulnerability scan requests targeting Wordpress will get from 90% to 9% on quiet webservers.