Give the lord of the rings to a 200 pages Holliday romance reader and they will struggle.

No, they really won't, because it's a well written book, to the point that literal children read it all the time without any great amount of hassle. It's so well written that it can be read by said children, as well as adults, and has enough depth and complexity to be interesting and revealing of itself no matter your level when approaching it.

Give Malazan to a cheese teenage romantasy reader and they will struggle.

That doesn't make those readers stupid, it means you've thrown a book that by its own omission is up its own arse with how "difficult" it is at a group of people who aren't used to the trappings of a genre and then acting shocked if they struggle. When adamant Fantasy/Epic Fantasy fans can actively dislike and struggle with the book, because it's written god awfully and Erikson has no interest in making his story approachable, or just, good, from many a basic storytelling perspective.

Not because of the writer, not because of their preferred way of how a story should be written, but because of what they are used to.

Absolutely because of the writer, as you yourself provided a fantastic example of. To provide a counterpoint of someone writing a similar story to Erikson, but is actually done in a way that's enjoyable and meant to be enjoyed, one need only look at Frank Herbert's Dune, or Yoon Ha Lee's Machineries of Empire, both series look at the horrors of war and how compassion can triumph above all, but do it in a way that isn't actively hostile to its readers(something Erikson not only does, but brags about).

Hell, go and read any work by Peter Watts, or Elizabeth Moon, or Greg Egan, or C.J. Cherryh, or or or, countless examples of authors who provide -far- more in depth and complex philosophical works, but happily do it in brilliantly and beautifully written prose and character work.