I'm not familiar with the design philosophy of the original protocol, or if they decided the protocol should or should not attempt to impose any kind of a security model, but given that the project grew out of the same kind of ethos as HAM, I'd bet it was never a high priority.
That said, these exploits make the mesh untrustable and should be mitigated. If the devs don't want to implement fixes, maybe the answer is stripping out all encryption entirely and adding a plugin system that allows anyone to implement a solution themselves. They could even move the current broken implementation to a plugin and make it the default.
In the future, they'd only ever need to handle requests for maybe new entry points or tweaks to the plugin system.