XS is the "same" hack all the time, though. A Perl library that can only be called in list context in Perl 6 is a substantial change from Perl 5. I've seen much smaller changes trash code bases before. I've got real code that would be affected by that. (I also think the context idea was a misfire in Perl <6, but as long as it is there, I've sometimes written code that had to deal with it.)
That said, why is it impossible to call a Perl 5 function in scalar context? Shouldn't the equivalent of Perl 5's "scalar" be fairly easy to implement? It may stink to have to manually use it, but it wouldn't be very often, and it's easy enough to use any of half-a-dozen Perl 6 features to minimize the syntactic hit.
The same could be said for XS. And I do think that all of Perl 5 is relying upon that.
> There's still no sane upgrade path here.
FWIW, my blog post https://www.perl.com/article/an-open-letter-to-the-perl-comm... touches on that subject.