It seems that Google provides six and a half years of software support for new hardware - from the date that that hardware is introduced. Thus, should you buy a Chromebook five and a half years after its introduction, you'll only have a year of updates/upgrades to look forward to.
At least a normal laptop can be upgraded to some form of Linux/FreeBSD/whatever when its supplied software expires. I've no idea how easy this is with a Chromebook, not having played with them. (They fetch surprisingly high prices on eBay - especially compared with, say, the old Eee PCs which have taken over my sitting room and much of my cupboard space...)
Probably still more secure than an unpatched version of some other operating systems, but something to watch out for.
More detail, including how to find out how much life your Chromebook has in it, in this article:
https://www.theregister.co.uk/2019/08/22/buying_a_chromebook_dont_forget_to_check_when_it_expires/
(Warning for the squeamish - the word "expiration" is used by Google insteady of "expiry." Approach with caution unless you're one of those fortunate souls who can cope with "going forward," "at this time" and similar horrors without having to grab a sicky-bag.)
