This does not explain why % does not interpolate...

I think it shows why making hashes interpolate would be a bad idea (given the current state of affairs), which amounts to the same thing.

... rather it shows that the choice of % as a meta character in printf is a bad one given that % is a sigil

Unfortunately, it's hard to make all those design choices up front, and now we're pretty much stuck with them. Larry could decide to change the printf escape character for Perl6. It would make it difficult to mechanically translate Perl5 programs that use non-constant printf format strings to Perl6, but maybe he'd find that acceptable. Changing the hash sigil at this point would be a much bigger problem.

printf '%d\n', $x;

To make this work, you'd need to have printf itself interpret the \n escape. That could be done, but it makes me nervous. Newbie programmers seem to have a hard enough time understanding escape sequences without muddying the water this way. I can just hear it: "Why does printf 'foo\n' work, but not print 'foo\n'?" Plus, if you actually wanted to printf a literal backslash, you'd need to put four backslashes in your code. Yuck.

Maybe there could be a new printf directive for a newline.

printf '%d%n', $x;