Parser combinator libraries have progressed a lot since then and, having used both styles extensively, I'm much more eager to pick up one of the more modern designs. I would expect a Boost library to reflect the state of the art not only in language features used, but also in API design for the domain.
Do you have any C++ examples where this is the state of the art? Do you have any examples of current literature? Declaring monadic parser combinators to be the state of the art when Haskell is mostly relegated to academic programs is kind of ostentatious, for lack of a better term. The thing about Haskell is that it's had time to gain ground but it just hasn't because people aren't interested in it. I don't mind exploring the design space of transforming parsers but I don't think there's anything particularly outdated about Parser's approach. If anything, I think the way you compose parser_interfaces in this library actually would be the natural transformation required to form the monad in the first place. - Christian