Would a new OS that included baked-in memory protection not have a similar performance hit to running Enforcer? and to even include the option of memory protection, all parts of the hypothetical OS have to be built with this in mind, meaning no shared memory space for IPC etc., thus slowing things down. Having an option to disable it is fine, but then all the IPC needs to be rerouted to shared memory in order to gain a little efficiency - otherwise why not leave memory protection on all the time. Turning it off won't give you the full speed benefits of not having the option at all, so you're already losing the edge on responsiveness that AOS has.
There's always NetBSD
that has reasonable hardware support and memory protection.