So that solves it for OP, but not for every single other person out there who's not as tech proficient and relies on iCloud for backups.

Which is not an unreasonable thing at all considering it's literally marketed as a storage solution for your photos, and a top of that even encourages users to store originals only in the cloud.

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".