The policy you want isn't "warnings as errors", though. It's "your PR must not add any new warnings (without a darn good reason)". That's not necessarily a reason to stop the build, and it's something that accounts for "I know there's a problem here but I won't (or can't) fix it immediately".
I'm one of the people against adding compilation-wide warning flags, for a few reasons:
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The same warning might fire in multiple files, but only be spurious in some of them. Turning it off compilation-wide means you miss those other issues, including any that might be introduced in the future.
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There ought to be a way to silence any potentially-spurious warning within the language (think "add 'as Foo' to silence this warning" notes). I know we don't have that for a whole bunch of cases, but I'd like to see people push on the ones that are important rather than just giving up.
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It creates language dialects.