[squillion]

It's not about monads, it's about effectful code, which is represented by special types (e.g. IO in Haskell, Eff in PureScript). Effectful code can call pure code, but not vice-versa. Since a program will have to do something, the main function is always effectful, i.e. it returns an effectful special type. So you're right that pure code isn't evaluated until some effectful code is ultimately returned by the main function and executed (by a runtime or equivalent). However, in purely functional languages most code is pure, even though it's ultimately called by effectful code.

Monads and side-effects aren't intrinsically related. Simplifying, a monad is something with flatMap() - in JavaScript, Array and Promise are monads (kinda). What flatMap() gives you is the ability to chain things, which is useful to sequence side-effects so that they can be performed by a machine in a given order. That's why IO and Eff are monads.

[ElectricalUnion]

To me the issue is even simpler, not even about effectful code or monads, it's about the "issues" of lazy evaluation.

It doesn't really matter if you made a horrible uncomputable mistake deep inside a pure function, if you either discard or never use that mistake you would never notice it happened.

For example, nixpkgs (in nixlang) for sure isn't "strict pure FP" - you can do effects whenever you want - but will often have uncomputable evaluations in it. And you will only notice they're uncomputable evaluations when you try to use them. There's even a warning left for future wanderers for that:

    ### Evaluating the entire Nixpkgs naively will fail, make failure fast
    AAAAAASomeThingsFailToEvaluate = throw ''
      Please be informed that this pseudo-package is not the only part of
      Nixpkgs that fails to evaluate. You should not evaluate entire Nixpkgs
      without some special measures to handle failing packages, like those taken
      by Hydra.
    '';

I see that I formulated my thesis badly, conflating strict evaluation with monads, when as you pointed out they're not strictly related.