> there were also a lot of people saying "well, if you just write your whole application in ReaderT IO, you can get 90% of the effects juice for only 10% of the complexity squeeze"

I think 90% is an overestimate because you don't get any encapsulation. Maybe 50% is a more reasonable number. If you want 99% of the effects juice for 1% of the complexity squeeze, my effect system, Bluefin0[1], as well as the effectful[2] effect system

My recent talk "A History of Effect systems"[3] explains this in more detail, as does the "Why even use an effect system"[4] section of the latest Bluefin documentation.

[1] https://hackage-content.haskell.org/package/bluefin

[2] https://hackage-content.haskell.org/package/effectful

[3] https://www.youtube.com/watch?v=RsTuy1jXQ6Y

[4] https://hackage-content.haskell.org/package/bluefin-0.0.17.0...