Excessive fear of backwards compatibility: you make -Wall. People use it. You add a new check which will reject some shoddy code. “What if people complain that their project no longer builds with -Wall? Let's add a new setting…”. Repeat…

I've been wondering how hard it would be to get to the point where the defaults are rigorous and developers have to opt out with specific -Wno-… options.

"-Wall" actually means, "all warnings, as of the date we froze the definition of -Wall, which for all you know was 20 years ago, good luck."

Apple introduced -Weverything because suddenly changing the scope of gcc's -Wall (which indeed does not include everything) would break a lot of builds.

Microsoft VC++ has a -Wall warning level too, and it really does mean all warnings. Each major version of VC++ potentially introduces new warnings to -Wall.