I think some of the frustration you describe comes from all-or-nothing thinking about having a positive impact. This type of thinking is probably more common among EA people, especially those with the goal of "doing the most good you possibly can".

As lilly mentioned, the majority of impactful jobs are probably not at EA-aligned organizations. Just because you're getting rejected by all of the jobs posted by 80,000 hours doesn't mean you can't get an impactful job elsewhere.

Also, fierce competition and repeated rejection are not unique to EA jobs. Think of all the people trying to make it as actors, musicians, writers, or in basically any creative industry. Think of all the people trying to make a lot of money without the goal of earning to give. Even in less competitive industries, getting rejected by the first 30 employers seems pretty normal to me. Perhaps the only difference for EA-aligned organizations is that more of the applicants have perfectionistic tendencies.

It seems the main argument hinges on the assumption that animals, especially arthropods, live mostly bad lives. How certain are we that this is true? Yes, animals feel pain and suffer. But we also have positive experiences. If preventing the existence of billions of animals is thought to have a net positive effect, we're assuming most of their existence is in a state worse than death or non-existence. What is the evidence for this assumption?