I don't think that a language that was meant to compete with C++ and in 10+ years hasn't captured 10% of C++'s (already diminished) market share could be said to have become "kind of the default" for anything (and certainly not when that requires generalising from n≅1).
The problem is that the number of browser engines is n=2.
It has for Amazon, Adobe, Microsoft, Google and the Linux kernel.
It remains to be seen which big name will make Zig unavoidable.