I don't see much of a monopoly, nor any "moat" apart from it being recognised. You can already post preprints on a personal website or on github, and there are "alternatives" such as researchgate that can also host preprints, or zenodo. There are also some lesser known alternatives even. I do not see anything special in hosting preprints online apart from the convenience of being able to have a centralised place to place them and search for them (which you call "monopoly"). If anything, the recognisability and centrality of arxiv helped a lot the old, darker days to establish open access to papers. There was a time when many journals would not let you publish a preprint, or have all kinds of weird rules when you can and when you can't. Probably still to some degree.