It won't solve for people that only own an iOS device but setting up a Time Machine backup is aggressively recommended by OS level notifications for every macOS users.

A simple usb hard drive will actually do, no need for a NAS. The only action required to implement proposed solution is to check "Keep all data on this Mac" in both photos and iCloud Drive settings. And to be extra cautious add a second backup drive from another vendor (to be extra extra cautious don't use Time Machine for the second drive).

For the specific case of thoses that don't have a big enough internal drive they might need to store data on an external drive. But if you do have 6TB of pictures you normally should ask yourself if a RAID1 or RAID6 is not warranted at this stage.

In conclusion it's not a binary decision there is lot of room between "I solely rely on the cloud" and "never trust the cloud".