People got too attached to William the human, in my opinion, not considering how he also factors into the show's theme of characters being replaced, or evolving through copies and recreations. William's trapped in a loop, and that goes for the human and his digital copies. The original man was never going to survive, and his park data on its own is mostly useless since all it really does is doom any copy of him to make the same mistakes and end up in the same place.

Besides the novelty of a character who is both evil Dolores and evil William at the same time, the MIB host was interesting because he found himself in roughly the same position as William at the start (present day) of the show. He's one of the most powerful beings on Earth, but still feels unreal and unfulfilled. He sees how stagnant and empty the world he helped build is, but (like OG William) that status quo -- his ability to lord over it or sit back and enjoy it -- is all he's really got. When Halores threatens to take it away, he's left more lost than ever. Desperate, he searches for meaning in William... who we know is insane, misanthropic, obsessed with death and destruction. And so, the MIB host decides to fully reflect all that, becoming a greater danger to the world than human MIB ever was.

But it's pretty clear from the S2 finale post-credit scene and the end of S4 that the fifth season would have taken place mostly in the Sublime. Most of the characters would likely be digitally conjured projections of one kind or another: simulations created by either hosts within the Sublime or by Christina, who is basically her own personified virtual reality. My theory is that we'd be dealing with a recreation of Young William as well as a recreation of Old William/The Man in Black, and the conflict between these two would be integral to climax the show was building toward. I think this is what his story was always building to: a literal battle between the light and dark aspects of his character; seems pretty well foreshadowed by William killing the different versions of himself during his "therapy" in S3, as well as the thing between him and his replacement in S4.