How is any new developer hitting any of these "quickly"? I'm working on a mature production codebase with plenty of advanced Haskell, and we barely have any tagless final code - much less free monads. Even HKDs and monad transformers are relatively recent additions to our toolbox that we reach for when appropriate - most of the code uses plain old datatypes and one or two concrete monad types.

My experience of learning Haskell is that many production codebases use advanced features because they are big boons to productivity, but few learning resources make reference to them. In fact, Haskell's biggest problem is probably the gap in knowledge between simple, low-productivity code written by newer developers, and the kind of code that actually goes into production with all the optional advanced features. A budding developer can easily write a whole application that never goes beyond `IO` without issue.

And yet everyone in my intro to CS class in uni managed to pick up Java within a week or two. Haskell is a bit more involved than Python, but not more involved than most languages.