People keep falling into the same trap. They love monopolies, then are shocked when those monopolies jerk them around.
Everything published on arXiv could also be published on Zenodo, but not the other way around.
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.