Hacker News Reader

You won't find much support for an opinion like that on a site named "hacker" news, haha.

A lot of backup services (tarsnap comes to mind) prioritize scriptability, as I imagine many people run backups from cron or a systemd timer.

Then it’s less scriptable. Many of us would prefer a CLI-based solution for that reason

It doesn't have to be one or the other. The GUI would be for one-off or intermittent usage, and the CLI program would still come in as the primary tool for scripting or recurring use.

Would it really be simpler to have 100 lines of GUI setup and tear down, obscuring the most important part (the commands being run)?

Depending on the GUI, it seems possible. You can also verify input in a GUI

Both sounds good as opposed to instead.

GUIs for simple stuff and 99% of the population, CLIs for hackermans who need the advanced stuff.