My thought is that, in some ways, Landin would be disappointed that Haskell/Prolog/Scala/OCaml etc., are all implementing whole languages from scratch -- Landin's argument, as I read it, is that devising a new ISWIM variant should be about as easy as writing your own Lisp.

That said, it does feel like the ML and Haskell communities really have taken up that spirit, with their emphases on multiple implementations and relatively ease of extension.