That's true, but a fallacy. Because HKTs prevent a certain class of problems from even coming to existence. What they predominantly do is help library author's

That's true, but a fallacy. Because HKTs prevent a certain class of problems from even coming to existence. What they predominantly do is help library author's to easily re-use and combine other libraries and provide them to their users while allowing for much greater flexibility than without them. So yeah, that's not a day-to-day problem of a system engineer (which I and most developers are) but they prevent a lot of these problems from even existing in the beginning. In the end, both are very useful in a different way and I don't like to work in languages that are missing either of them.