The pain of breaking changes is proportional to the volume of code affected by the breakage. This usually means breakages early in the lifetime are easy because there's relatively little code. For mainstream languages that have a decade or more of widespread usage, a breakage is a big deal.

> I honestly don't really know of a story where a source-incompatible update killed a language.

Perl. I mean, Perl isn't strictly dead, but its share of the market plummeted. Python almost certainly would have suffered a similar fate if it weren't for the explosion of interest in scientific/numeric computing (which more than made up for massive attrition to other languages, including Go).