Even with UB holes plugged, C (and C++) are still unsafe, because there are many assumptions you might want to make that you can not encode in the language.

To get an example that's easy to understand: before the introduction of the 'const' keyword, you just couldn't express that some variable should never be changed. And no amount of UB sanitisers would have fixed this for you: you just couldn't express the concept. There's lots of other areas of these languages that are still in a similar state.

Eg there's no way to express that a function should be pure, ie not have side effects (but is allowed to use mutation internally).