Great post. Something like this probably qualifies as an axiom: "understand
you may be rewriting".
Another issue that I haven't seen touched on yet is the new CTO's ego and political motivations. I would guess that if a new CTO just came in and shot par for the course by keeping the legacy system running well, that CTO wouldn't have much to add to their resumé. It seems like it makes more slimy political sense to say "Everything sucked until I came on board and made it better"; especially if it's not true.