Right now, final still has an ABI impact, where it means "something that will never ever be subclassed or overridden". The Core Team guidance on actor inheritance is that it is not part of the proposal, but they/we did not shut the door on it completely. Allowing final lets us keep the door open in the ABI and language to change our minds if actor inheritance turns out to be very important, and we can ban it later on if we decide that Swift will never have actor inheritance.
Doug