If one day all of the bounty activities can be published and recorded using blockchain, then this kind of deception will be easier to find and then only honest projects will be left behind.
Here is the best sign that crypto hasn't come very far yet and indeed is in need for a killer-app! For bounties what we need is a platform that is able to do that (probably exists already, ETH or similar) and is easy to use (I don't think this exists yet!)! Easy means easier (or not much more difficult) than maintaining an excel sheet but with all your mentioned advantages (and not too expensive either)
It is a shame with all the bounty-scams and very time consuming to find appropriate projects and bounty managers for one that is willing to do bounties. But there is also an opportunity in here to get a real-world blockchain app that can be useful for everyone!