Also, you of course do not need purity to get monads to “work”. You can do them in SML or OCaml, for instance, which are not pure by a long shot; by the definition of “pure” which I employ, Haskell isn’t either.

Lack of purity is a defect which bites you whether or not you have monads: so if you're perfectly happy dealing with it in your existing code base (like most of us are), making explicit the “monadic” patterns in your codebase will not make it worse, and may improve things a bit.