OP asks:
"What happens when Apple no longer provides security updates for your Mini/Pro/iMac, etc.., computer? Do you just practice safe hex?"
Nothing much happens, really.
You just keep using it.
I've got a 2010 MacBook Pro that came with 10.6.3. I upgraded it to 10.6.8 (last update for Snow Leopard), and it still serves as my "main OS" on that laptop.
It runs fine and gives me no problems.
On my late-2012 Mini, I still use 10.8.5 Mountain Lion as my main OS.
Again, runs fine and gives me no problems.
Could the lack of available security updates cause trouble?
I -suppose- that in theory, it could.
But in my own experience of using Macs since 1987, I've never had a "security-related" problem that arose from using older OS's.