The ChromeOS use case is a strict subset of the use case of a normal Linux laptop. There is nothing stopping you from treating an Ubuntu installation as if it's as limited as a Chromebook. It's every bit as lightweight and can be every bit as locked down.
The hard problem of security is giving the user the power of a general-purpose machine without exposing them to the risks. "Don't run your favorite software lol" is not a valid approach to security. ChromeOS remains totally unsuitable for even casual usage, let alone anything serious.
Someone will, I'm sure, claim that the use of virtual machines is a solution. It isn't. The layers of virtualization in ChromeOS lead to atrocious performance, reliability, and functionality. I am not willing to tolerate half my programs living in a different universe from the other half, nor am I willing to tolerate uptime measured in hours.