I have a pre-Google Nest thermostat which I like quite a bit ... it's probably saved us thousands of dollars in the decade we've had it. Is this also destined for the Google hardware graveyard?
The alarms themselves should be able to work even without network connection, but you won't get to use any of the connected features, loosing all the "smarts" that they charged about $100 premium over other smoke alarms for.
It seems the current plan is to let the device reach their use by date before they shut off the servers, but Google being Google, who knows if they won't change their mind and decide to shut off the server before that.