The game’s not — and never has been — about getting someone to replace the Outsider, it’s about trying to rectify the cycle of suffering by dismantling the cult surrounding him. Purging a plagued world of its idol of chaos is a tentative step forward for the prosperity of society. The world of Dishonored is one where there are no happy endings, but dissolving the occult by removing what was fundamentally their god and giving him the mortality he was robbed of is undoubtedly a step forward. It all boils down to either choosing the lesser evil and doing what good you can in a corrupt world (sparing the Outsider), or sowing the seeds of chaos and destruction and forcing the world to face its consequences (killing the Outsider).There is far more nuance to it all, but that is a very crude way of summarizing it. I recommend you give the series more thought, or at least replay the games so you can see that for yourself.