> That's a ludicrous expectation will lead to developers being scared of merging changings, which will in turn lead to them hiding mistakes, refusing to talk through their code, and ultimately hacking things in to the codebase that "work" but are terrible solutions to problems.

Maybe, but it can just as easily lead to good practice. E.g. if you're scared to change a particular piece of code, that's a red flag that that particular piece of code is inadequately tested - and "am I scared?" is a much better metric than automated test coverage reports.