They flung them out into space hoping they'd land on planets with world-souls, because they didn't know which ones did or did not have world-souls. I don't think it's that far-fetched, especially with the "fluidity" of WoW lore, to think that additional Old Gods might be sent our way.
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Before Legion, one might have thought that a full-scale assault by the Burning Legion on Azeroth might have been a hopeless situation. I don't think more Old God involvement is the craziest thing every.