When Microsoft updates or issues an 'end of life' date for a version of Windows it can be pretty big news. Only a few days ago we learnt that Windows 7 will not be installed on new consumer PCs from October but it's still not had its card marked for business and enterprise machines. Also we have the end of extended support for Windows XP to look forward to in April. However other OSes and computer systems stop being supported and become more vulnerable to attacks, viruses and exploits - even your shiny new "always up-to-date" Chromebook.
Your product model is considered obsolete
Google has published an official table charting the 'end of life' of many popular Chromebook (and Chromebox) models. The table applies to only ChromeOS devices in the hands of Enterprise and Education customers but we expect that consumer bought Chromebooks should get the same length of support.
You can see from the table above that there are Chromebooks that, unless support is extended, will become obsolete by July next year. The majority of other models have one or two years more. The word 'obsolete' is from Google's definition of what will happen after the Chromebook end of life date. See the quote below;
"When a device reaches End of Life (EOL), it means that the product model is considered obsolete and will no longer receive full support from Google’s Enterprise team. Additionally, Enterprise customers using devices that have passed their EOL date may find that they cannot manage their devices as expected using the administrator control panel or leverage new management features released."
Other info regarding this EOL date includes the general principle that ChromeOS devices will get support for a minimum of four years from their launch date. Also Google notes that "The End of Sale date is controlled by the OEM (manufacturer) of the device model and has no relation to model’s EOL date." So a bargain 'new' Chromebook might have a shorter useful life than you had expected. At least this is better than the Google Android device updates situation where vulnerable versions of the OS, as sometimes installed on new devices, never get updated.