Does arXiv prevent your work from getting scooped, or the opposite? On the one hand, it seems like a preprint can discourage dishonest reviewers or editors (who get to see new work before anyone else) from running off with ideas since you have proof of originality with a preprint. On the other hand, a preprint puts the ideas out there to the world at large. What do you think is the safest approach, submitting to journals with or without a preprint?
Preprint services prevent scooping.
Every preprint service puts a date stamp on every document that is published there (and in many cases a DOI as well). That's an effectively unmodifiable record of the time when something went live, at which point nobody can republish your result elsewhere with an earlier date stamp with any reputable publisher.
That said, preprints will not prevent competition for the generation of follow-on research based on a result, nor will it prevent somebody from claiming independent discovery of the same result. Those, however, are just a normal part of doing business in science.