I would really love to buy a Pixelbook (or another Chromebook) because I really like the software stack. But the fact that Google never updates the kernel on those things (you're stuck with whatever kernel version was on it when you initially purchased it) is a major reason for me to not get one.
On my Linux laptop I can always run the latest software. On my Macbook Apple will stop to provide software updates at some point, but in practice the hardware is probably going to die before that anyway. On a Chromebook parts of the software stack are never going to be updated. And while most of the users probably don't care about the kernel version itself, a lot of Chrome OS features are dependent on the kernel version: with an older version you're going to miss some features (such as being able to run Linux apps), although the hardware itself would be capable of supporting them.
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.
On my Linux laptop I can always run the latest software. On my Macbook Apple will stop to provide software updates at some point, but in practice the hardware is probably going to die before that anyway. On a Chromebook parts of the software stack are never going to be updated. And while most of the users probably don't care about the kernel version itself, a lot of Chrome OS features are dependent on the kernel version: with an older version you're going to miss some features (such as being able to run Linux apps), although the hardware itself would be capable of supporting them.