Except the energy that has been burned in the last 9 years used to create them. The security comes from the need to spend as much energy to 'undo' them.

Where do alts (esp non POW alts) get their value? Because people want to trade them to get more bitcoin, which has value as per above.

You can keep repeating this circa 2014 argument 'not backed blah blah', but as yet it has not gone to 0, and likely never will.

Minimum value would be the work/energy needwd to create blocks that enable safe storage and avoid double spending. So there is that.