Why is the option to reboot Vigor routers Using default factory configuration offered so readily whenever a relatively major configuration change is made?

Only a single radio button change is needed to lose the lot. IMO there should be dire warnings if that option is selected. Better still, do not offer the option but let the admin user navigate to the System Maintenance > Reboot System page if such a drastic reset is required.

In my case all I did was change one LAN port to a different VLAN and then accidentally select the nuclear option.

Does anyone disagree?

Agree completely. But look on the bright side, you wont do that again in a hurry!!

My old 3320's firmware had the option to backup config to USB every time there was a change/daily - is that an option on yours?

I had a full config backup file so restoring the settings wasn't a problem but the router is managed remotely and the default WAN settings don't quite work with the Cisco that controls the leased line, so I was locked out. Embarrassing.