Ok - you NEVER have to take down YOUR DNS server for maintenance of the DNS server itself for patching OR the OS?
If I time the amount it takes to restart the DNS server (much like how the daemon is restarted after updating it):
So, I guess as far the rare updates (I think I only ever saw one last year), there might be a downtime of 0.008 seconds, which isn't really long enough to miss all the standard three DNS requests most resolvers send out.
Now, if this wasn't on the same system that provides Internet routing to the rest of my network, I'd probably invest in a Bird setup for reliability of certain network services. Which would be trivial on a couple of Raspberry Pi Zeros to provide the network uninterrupted DNS resolution (among other network services).
As far as the hardware restarts go, according to my router data, it's been rebooted four times last year (I believe two were for system patches, two were due to me physically moving the hardware in the house). I am not seeing the "DNS is prone to going down" problem here.