Non updated full nodes could conceivably put the entire network at risk in the future.
I don't agree. If a user isn't accepting transactions on a node or mining with it, there is no particularly urgent reason to upgrade. There are plenty of upgraded nodes on the network now.
The reason the notices encourage people to upgrade urgently is not because all actually need to, but because figuring out if you really need to or not is hard, so the best advice is for everyone to do it.
Updating to 0.16.3 from 0.16.0-2 for a non sophisticated Linux user may be prohibitively complex. Add to this the fact that a neophyte might not even become aware of the need to update at all in situations like the one encountered wrt the 0.16.3 update, and the urgency of streamlining the update process for end users seems to me to be pretty high.
That sounds like a reasonable concern but it has to be counterbalanced against the risk of a malicious update being deployed. The fact that upgrades take a while makes me feel confident rather than frightened.
It's like the advice goes, "if you never miss a flight, you're probably wasting too much time in airports". If we never had reason to wish updates went faster, we'd probably be excessively exposing users to the risk of a bad update.
If there ever were a really serious issue where updates had to happen or else, we'd be probably advising people to stop accepting confirmations and to potentially to turn their nodes off.