The problem as I see it is that a lot of those bounties are $2,000 for $20,000 worth of work, especially since they end up getting attached to the most gnarly bugs whose true solution involves deep refactoring or building essentially whole new products. Thus the maintainers need to be willing to support the features indefinitely, and they need to agree with the architecture of the solutions, and quite rapidly you start to understand why a little cash is not sufficient to solve the most intractable problems.