I've set up a few Linux installations for people who only use their computers as Facebook and YouTube machines, and I haven't had a complaint. They also wouldn't be able to break their systems if they tried.
I'm of the opinion that if ChromeOS would fit a user's use case, then so would Ubuntu with Firefox or Chrome, most of the time.
Those same Linux systems would fit my needs as a developer with only a few small changes.
Security, simplicity, power and ownership don't have to be mutually exclusive. You can have a simple and secure computer, and also have power over your system and own your hardware.
Yet at the same time it makes it possible for the user to "grow" and make use of more advanced features of the system for creative endeavors.
On the other hand on a locked down mobile device or chromebook, there is not really any room to grow and be creative, it's only good for consuming content.