On iOS macOS the kernel version isn't an issue as Apple ships OS updates with the latest version of Darwin.
On Chromebooks the kernel version doesn't matter if you only use basic features. But for more advanced features the kernel version determines which features are available. For example, Linux app support for Chrome OS isn't available on kernel 3.14 and older.
Chrome OS devices usually ship with a certain kernel version which isn't updated later on. So you could have a device where the hardware would be perfectly capable to support a given feature, just to not be able to use it because the vendor refuses to update one of the core components of your system.
I don’t understand this argument.