There are 2 ways to get bronze: 1. Fly hardly ever in premium cabins (you get PB anyway and you are hardly ever using PB). 2. Fly a lot down the back (for 25 sectors a year you can have PB for all it's worth). I think the idea that the bronze card holders are somehow cramming the PB queues to breaking point is overblown on here. My regular route EDI-LHR is absolutely chokka with gold, silver and their OW equivalents.

As has been said: the solution is simple and costs nothing and would make (almost) everyone happy without having to remove PB from bronze!