I don't see why they couldn't extend support to indefinitely. A Chromebook is mostly just a front end to Google's webapps. They don't need much beyond security updates.
The hardware isn't great and the build quality is usually similarly low, but it's not meant to be a Ferrari. Don't make crappy, bloated JavaScript and there's no need for new hardware until a device breaks.
Google doesn't really care about selling new devices. They get their value of getting people hooked on the service and the service can run on old devices much the same as new ones.
Really Google should use the backside of Moore's law to pick a particular performance target and let newer Chromebooks become less expensive over time as it's cheaper to get the same performance over time.