There are two problems with this. One, it's another physical device to manage which is a pain in the ass. Two, what happens if the external HDD breaks?

Nothing bad, if you've done backup properly (i.e. three copies; two local, one remote.)

Mirrored drives on your desktop plus an account with Blackblaze (~$5/mo. for unlimited storage), and you're covered. Done and done.

It reduces the triplicate pains in the ass caused by trying to manage three physical devices bought from Apple....

However, note that if you just move all your photos to an external hard drive, you don't have a backup.

No it doesn't. You still have those devices. They don't go away. Now you have expend time manually copying your photos to the device.

You can use backup/sync software to copy stuff across automatically. Otherwise, I didn't say your pain-in-the-ass idevices went away, I just said you could reduce their pain-in-the-assness.

Backup the HDD and you'll be fine. Copy and paste and walk away. Personally, I have private pictures that I would never want put on any cloud service. Nothing in the world is 100% protected. There could be a bomb that drops on the building housing the servers. A fire could burn down your house/dwelling. It happens.