And it doesn't decay, so while nuclear waste will lose 95% radiotoxicity in just 100 years, mercury and arsenic will last forever!
When did you last hear Greenpeace chaining themselves to rails to prevent transport of mercury and arsenic waste to these mines? Never. Because it's not radioactive, it's not scary, even though it's much more toxic.
People just trust this salt mine will last forever, and they're right because this is what geology says.