(Some) Chromebooks, tablets, and other devices…are subject to Google’s End of Life policy:
“When a device reaches End of Life (EOL), it means that the product model is considered obsolete and automatic software updates from Google are no longer guaranteed.
“Google will provide advanced notice of a model’s EOL Date on this page as soon as it is launched, giving our customers time to make purchase decisions.”
Some past their prime…
Asus Nexus 7 (2012) – Jul 2015, Android
Google Cr-48 – Dec 2015, Chrome
Samsung Chromebook Series 5 – Jun 2016, Chrome
Acer AC700 – Aug 2016, Chrome