If OW2 is a full-on replacement for OW1, something about their current scheduling has to change(or it's got to be slated for 2nd half of 2021 or later, in which case Blizzcon 2019 seems like a weird time for a reveal). Because with the current "confirmed" heroes in the pipeline and the Overwatch League, there isnt a "free" time to release it aside from fall 2021. And I dont see them dropping OW2 and then either continuing to update OW1 or trying to keep the OWL running on OW1. As soon as OW2 hits they need everyone hyped to get in on it and not sitting on OW1.
And this is why I'm in the "OW2 is a PvE addon/expansion/spinoff for Overwatch" camp until we learn more, because based on what Blizzard has out there atm there just isnt a place for OW2 to fit into their current plans, and any attempt to keep both games running concurrently would just doom OW2.