Ignoring undefined behaviour

I disagree deeply; the people I see arguing that compilers should only care about the ISO standard are a disjoint group from those who say that the standard should expect downstreams to define more behaviour than ISO does. It makes a huge difference, because it's a minority group, who just happen to have positions of power w.r.t. open source C compilers - note that many of the proprietary C compilers don't make the same arguments around "ISO says it's OK" - and your position is a lot like saying that because you hold an opinion, Google as your employer must agree, and that it's unhelpful to view your opinions as separate from Google's.

In practice, if enough of the people who care were to get involved with LLVM and GCC maintenance, and write and enforce documentation for what LLVM and GCC do when ISO says "UB", "IFNDR", "US", and similar terms for "ISO doesn't have a view here", it'd stop being an issue. But that would involve a bunch of people who aren't interested in compilers taking control of compiler projects, and this is an underlying weakness of open source - only people who are genuinely interested in something tend to take control of that thing.