These were examples of the choices one has to make when using Haskell; it is far from exhaustive. The point is that IMO Haskell undervalues consistency and standardization.
Haskell has its weaknesses, e.g. the lack of quality tooling available as compared to mainstream programming languages.
But I disagree that Haskell is complex, at least as a criticism. When expressing concepts of a similar complexity, I find Haskell to be particularly concise and expressive as compared to most other programming languages I am familiar with.
And if Haskell undervalues consistency and standardization, I would like to know as compared to what? The only programming languages that I know of where there are not many reasonable choices for e.g. a testing framework are those that either (a) haven't been around that long, or (b) haven't seen wide adoption.
Haskell is many things. Simple is not one of them.
Language extensions are idiomatic Haskell. There is only one mainstream compiler used everywhere. Widely used extensions are a natural result when the compiler, as a testbed, can outpace the language standard.
Enabling extensions is as trivial as including a standard library package. (In fact, the extensions are often better documented than the standard library, as you hint to.)