rc.conf: system configuration
inetd: specialised starter for internet tools
There is no "switching from inetd to rc.conf", both are really different tools: Inetd fires up a service only when it is requested to be run, while services started via rc.conf are up right after the system boot. And rc.conf isn‘t only a starter for services: hostname, keymap, network configuration stuff etc. is configured there. Internet services started via rc.conf are daemons, they are running all the time. Services started via inetd are stopped after the job is done.
So: Yes, we need rc.conf. If you need inetd depends on your use case.