Yes, people can get crazy with inline anonymous function chaining/composition, and that can quickly get out of hand in terms of maintainability and readability,

Yes, people can get crazy with inline anonymous function chaining/composition, and that can quickly get out of hand in terms of maintainability and readability, but deeply nested imperative loops is often much, much worse to debug and understand, because the intermediate steps are not nearly as explicit as in a functional chain/composition that simply takes data and returns data at every step. Regardless, these are simply cases of people writing bad code, and nobody is claiming map/reduce/filter is a panacea for bad code. Functional composition/chaining works best with small, well-named single purpose functions that compose/chain together into more complex functionality (with appropriate names at every non-trivial level of chaining/composition). You can't easily compose/chain imperative loops this way (at least not without wrapping them in functions that take data, and returns transformed data, by which point you might as well use map/reduce/filter to transform the data to begin with to get rid of the impedance mismatch).