Libraries are where unsafe belongs. Having unsafe be an explicit feature is immeasurably better compared to every line in the entire program potentially being unsafe.

Opting out of safety is fine, but safety must be opt-out to be effective, not opt-in. A standard, enforced way to tell readers where the dragons lie is super helpful.

Right, note that I said safe Rust, which is the vast vast majority of Rust out there. This is a major improvement over, say, C, where any line of code could be potentially unsafe.