Setting the serialization format issue aside, why is the D-Bus broker an issue? Why was it designed that way in the first place if all we need (supposedly) is

Setting the serialization format issue aside, why is the D-Bus broker an issue? Why was it designed that way in the first place if all we need (supposedly) is a socket and a simple request/response protocol? If it wasn't needed, why does D-Bus have it? And if it is needed, what will replace it when systemd switches to Varlink?