100% coverage does not protect you from bugs when you develop. It protects from regressions when you update and refactor.
100% coverage also means having a fairly simple and testable code. It’s a big plus when one wants to reproduce and fix a bug.
Therefore, coverage is a good indicator of quality.
In legacy, 100% may be impossible, but 70-80% is achievable also on legacy code, especially with a bit of refactoring. One should start with the 20% of the classes that are changed more often. It is possible to datamine the repository to spot them.
The only real problem is to get the customers on board if they cannot distinguish between good quality and pure garbage.