This is my personal theory as to why mecha-Otis was killed so suddenly and so pointlessly. If we had 'living' Otis exist at the same time as 'Endless' Otis (well, they DID exist at the same time for like 400 years but I guess they want us to think about it), it would kind of make it unacceptably clear how what they're going for emotionally in living memory is NOT what's happening lol. They had to make sure he wasn't around in the hopes that we just wouldn't think that hard about it.
