I see SKB as more something to allow the First Order to be an actual threat for the later films. Because without that shot, the New Republic Fleet should, realistically, be able to pounce on the First Order really easily given the New Republic having had three decades to rebuild the galaxy and their own navy with at least a sizable amount of the resources of the old Galactic Empire, or just the resources of the Old Republic and part of the old Separatist worlds, while the First Order is scraping around the Unknown Regions trying to built stuff to give them an edge over the large conventional warfleets of the Republic.
The Death Stars might have been very good at causing fear and blowing up planets, but Star Killer Base actually performed a functional military grade strike by knocking out a key world and presumably the majority of the Republic Navy with the several planets destroyed in that one shot. Potentially all those worlds were the major fleet bases and thus took out the major capital ships, since those wouldn't usually be out on patrol when the Republic is basically at peace.
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The Death Stars might have been very good at causing fear and blowing up planets, but Star Killer Base actually performed a functional military grade strike by knocking out a key world and presumably the majority of the Republic Navy with the several planets destroyed in that one shot. Potentially all those worlds were the major fleet bases and thus took out the major capital ships, since those wouldn't usually be out on patrol when the Republic is basically at peace.
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