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
They say Fud is in the official group, and that I think is stupid action that can affect investors and even people who will invest. The project is successful because there are investors, but when investors want to invest, they see that there are people who spread Fud and of course the investors will think longer or not invest in the project.
and what will happen? the project failed, bounties did not get paid and bounty hunters were increasingly spreading FUD.