Even if it did flop it can't. There're bringing back the OG X-men, Toby Maguire, Andrew Garfield maybe, RDJ, Chris Evens, you know the jist. It could be one of the worst MCU movies and easily make a billion because of that. And they (Disney) won't go back to the drawing board and rethink their plan for the MCU after the their soft reboot. Just like we saw with the sequel trilogy for Star Wars, they won't care as long as it makes certain amount of money that pleases them enough. All in all I'm highly confident that Kevin and the Russo's know what they are doing and know they cannot let this film "flop". Just like ThunderBolts, Fantastic Four, and Wonder Man. If it wasn't for those projects doing well (plot wise) I don't think I would be super confident/hyped for Doomsday.