You are very right, the only solution to bounty hunters getting low rewards is what you've stated in your post, there should be limit to numbers of participants needed for specific bounty campaign, so that rewards will be fair for few hunters that manage to get in.
I think bounties are open to everyone and the minimum rank of jr.member is fair because not all bounty hunters work well and sometimes they even do or spread FUD on the project.
I have never seen a bounty hunter spread fud when they promote a project,
I only see many participants and small rewards for bounty participants,
indeed there are bounty managers who have bounty participant limit rules, but many bounties still haven't used this rules