> I give you, that "%" is bad for named arguments, but why wouldn't you just improve that then?

Perhaps they should. I'm not making a case specifically for the proposed strategy (interpolation), just that if they are going to implement it, that it took this long is odd, and somewhat laughable. Personally, I think the named string format is superior to this new feature in many respects except the most simplistic of cases, but I don't often write python, so my opinion may be colored by that.